This is an intriguing question. The answer is design, not technology.
Some digging into early history of printing and the handwritten pre-history
of printing is needed to see that design, not technology, was paramount.
In what follows, I omit many details of importance to type historians and
I simplify the overall picture but believe it is mostly correct.
1. Origin of Italic.
The earliest roman typefaces were cut in Italy and imitated humanist handwriting
of the era. Humanist book script was a clear, legible, upright style of handwriting
that Poggio Bracciolini, a scribe and scholar, developed circa 1400. The minuscules
(like lowercase) were based on Carolingian handwriting circa 800 AD.
The capitals of humanist book script were derived from Roman inscriptional
capitals, which had survived in Rome and many other places in Italy, and
which humanist antiquarians and architects studied, copied, and often tried to
construct in terms of geometric principles. The humanist scribes like Poggio,
Niccoli, and their later followers, reinterpreted the inscriptional capitals in
handwriting but didn't alter the forms by much, except that they were handwritten,
not inscribed in stone.
German printers, Sweynheim & Pannartz, working in Italy around 1465 in Subiaco
and later in Rome, cut perhaps the earliest roman types, to appeal to potential book
buyers who were likely to be humanists who preferred humanist handwriting. The
decisive roman typeface that influenced all later roman types was that cut by
Nicolas Jenson and first used in 1470 in Venice.
But, before printing, and after, there was a variant of the humanist book script,
a "running" or "cursive" style written by Niccolo Niccoli, a friend of Poggio.
Niccoli was not a professional scribe like Poggio but a collector and reader
of manuscripts, many of which he copied himself from borrowed manuscripts,
so he write fast. Speedy writing made his letters slant slightly and caused some
changes in shapes, for example to 'a' and 'g'. Niccoli's capitals slanted slightly,
but were not different in basic form from the upright, and didn't slant as much
as the minuscules.
As humanist handwriting gained popularity among the educated elite and the
rising class of new wealth in the Renaissance, the literati and "cognoscenti"
so to speak, adopted humanist handwriting not only for professional and scholarly
reasons, but for social status, it was cool. It was almost like you couldn't get
a date if you couldn't write humanistic script.
2. Aldus and Griffo, 1501.
The first italic typeface was cut by Francesco Griffo for Aldus Manutius and
appeared in a book of Vergil in 1501. Thirty years after Jenson's model roman.
The Aldine Italic lowercase was cursive, that is, with shapes influenced by fast
handwriting, but the capitals were upright. Griffo cut roman and italic type with
equal facility. The only technical issues with italic were that, as handwritten,
there were lots of variant forms used by the scribes, and Griffo was able to
reproduce those in type, but later typefaces tended to have fewer variant letters
because they added cost in cutting, casting, composing, and distributing type.
Printers began to economize to compete.
3. Later 16th century italics.
Griffo's italics were imitated by other printers, but in the 1520s, a different style
of italic appeared in print. It was based on a refinement of Niccoli's cursive
handwriting that had been adopted by the Papal Chancery decades earlier.
It could be written quickly but remained legible, and the Chancery scribes
endeavored to make is elegant as well. As printing replaced traditional scribal
jobs in reproducing manuscripts, except in official church and civil chanceries,
some scribes specialized in fancier forms of writing. The Papal "chancery cursive"
was more sophisticated that Niccoli's original hand, which was practical, not elegant.
So, then there were two kinds of italic in print, and they were used in competition
with roman, not in subordination.
In the late 1520s and through the 1530s, printers experimented with italics.
There were several variations from different printers in different countries.
It was a great period of aesthetic variation. As a few styles became more
popular, their shapes, spacings, and weights were harmonized a little more
with roman types. Around 1539, there two important advances. The first was
inclined capitals to mate with the cursive lowercase. The slanted capitals had
the same shapes as the roman, but were slanted and optically adjusted to make
the stems and strokes harmonious. The other big change was that italics began
to be used as subordinate companions to romans, not as competing styles, and
this relationship became established by the 1560s. Some say earlier. The first
type "family" of roman and italic cut by the same punch-cutter was around
1568 or 1570, by Guyot.
4. 17th, 18th, and early 19th century italics. As with most other type, each
new generation of styles imitated earlier types, not handwriting. All the way
from Garamond to Jannon, to the Dutch styles of Van Dijck, to Caslon, to
Baskerville, to Fournier, to Bodoni, to the Didots. Again, it was accepted
style that made italics truly cursive, not slanted romans. But, importantly,
italic capitals had always been slanted romans ever since the early cuts
by Garamond around 1539. So, punch-cutters could slant type quite
effectively when the shapes were called for, but there was no tradition of
merely slanting lowercase.
5. The Grotesques.
In the mid 19th century, English typefounders developed several versions of
sans-serifs. The earliest, around 1815, from the Cason foundry is known
from a specimen, in capitals only, but appears not to have been used much.
Later sans-serifs, called "Grotesque", came from the Thorowgood foundry
in the 1840s. These were in upright form only. They began as display faces
but later in the century, adopted by German foundries, who used the term
"Grotesk", were cut at smaller sizes for everyday printing, like labels, schedules,
etc. It appears that in the 19th century, typefounders didn't think of cutting italics
for Grotesques. Italics weren't needed for jobbing work.
6. 20th century. I'm not sure when the first slanted sans-serifs were cut.
Certainly by the first decades of the 20th century, when Morris Benton
designed "italics" for Franklin Gothic at American Type Founders, and
the Bauer foundry produced slanted versions of Venus Grotesk. It's
worth noting that for many grotesques, there were no italics even then.
Slanted versions of grotesques became more common and popular
in the 1920s and 1930s, when the avant-garde typographers and "neue"
typographers adopted grotesque as the type style of "our time". Because
there was a tradition of italic in seriffed type, once modern typography
went to sans-serif, it inherited a need for something slanted. However,
the connection between italic/slanted type and handwriting had long
before been broken, so there was no handwritten sans-serif to inspire
sans-serif type. Sans-serif types were sculpted forms long divorced from
handwriting, so there was no tradition, and simply slanting the upright form
was deemed adequate. As late as 1926, Stanley Morison wrote an article
on "Toward an Ideal Italic" in which he argued that for seriffed typefaces,
a sloped roman would be ideal, but with fastidious Anglo-Saxon sensibility,
he didn't even mention slanted sans-serifs. Nevertheless, when Monotype
commissioned Eric Gill to design sans-serif type in 1928-30, Gill drew an
Italic, which, notably, was based more on traditional cursive writing and
stone carving, than on merely slanted roman.
In the 1950s, when the neo-Grotesque styles of Univers and Helvetica
were released, the need for italic was evident, as it had been since the
1920s, so those sans-serif families were provided with slanted versions
loosely called italic. One technical issue did arise that appears to have
prevented Univers from becoming the dominant sans-serif of our time.
Frutiger designed Univers with a fairly steep angle of 16 degrees.
This was OK for phototype, which was the intended medium, and also
OK for foundry type by Deberny & Peignot, but when Linotype wanted
to put Univers on the Linotype line casting machines, they had to reduce
the italic angle to 12 degrees for technical reasons. The steeper angle
could not be achieved in Linotype matrices without excessive spacing.
Frutiger said the steep angle was important to distinguish roman from
italic and to give italic its own snappy, strong effect, but the Linotype
machinery couldn't handle it. Hence, Linotype adopted Helvetica as their
dominant sans-serif, with its lesser slant for italic. There were, of course,
other technical issues involved, involving letter spacing and so on, and
the differences between foundry type, Monotype, Linotype, and phototype.
In a parallel path, some sans-serif types did offer cursive italic companions.
The first, to my knowledge, was Eric Gill's Gill Sans. Gill was a traditionalist
in some ways and had been a student of calligrapher Edward Johnston, so
he understood the historical origins of italic and, having a strong minded
character, he didn't hesitate to impose his views on italic, and Monotype
went along with it. Some 40 years later, Hans Ed. Meier made his Syntax
Italic a true cursive, although for reasons of humanist origins and modern
legibility, he kept the older humanist forms of 'a' and 'g' even in italic.
Lucida Sans italic, of 1986, went back to humanist cursive for its italic.
And later sans-serif designs by some Americans and several Dutch designers
also had cursive sans italics. Today, a cursive humanist sans italic seems
like a no-brainer. Many young designers do it.
Frutiger always liked a strong slant, even with his later "Frutiger" design.
Its italic was a slanted version of the roman, with subtle visual adjustments,
not a simple mathematical slanting. Later, a version of Frutiger was brought
out with a cursive italic. But, it turns out, that was designed by someone
else, a younger designer, and Frutiger never liked it. So when an even later
version of the Frutiger family was brought out, Linotype reverted to the
slanted form of italic, as Frutiger wished.
7. In digital types.
In the early days of PostScript and Apple Laserwriters, Helvetica "italic" was
merely an algorithmically slanted roman, slanted automatically in the printer,
not a separate font. This was to save space in the computer memory. Extra font
outlines would have made the printer too expensive, back in 1985.
A variation happened with Lucida Grande in OSX. Apple licensed Lucida
Sans roman and "italic" from us, but their font "expert" didn't understand the
difference between true cursive and slanted roman as italic. We delivered to
Apple the true cursive Lucida Sans Italics. But later, their user-interface
designers balked, and when they were finally permitted to talk with us (Apple
liked to keep things compartmentalized) they told us they wanted something
simpler looking, like a slanted roman, although their "expert" never told us
that. We made Apple a set of slanted romans for italics, but that time, they
were so close to launching OSX in 2000, they had no time to include slanted
versions of Lucida Grande, so italics were just left out entirely, Some users
complained, but Apple never explained why italics had been omitted and
never got around to offering Lucida italics, whether slanted or cursive.
So, there is really no big technical problem with sans-serif italic. It is mainly
an issue of origins, traditions, and aesthetics. Occasionally, some technical
problems arose, like with Univers on the old hot-metal Linotype machines,
or slanted Helvetica on the first Laserwriter. But, in the main, the different
developments and adoptions of true cursives versus merely slanted italics
for sans-serif faces are matters of design, both in the creation of the forms
and in the tastes of typographers and graphic designs who use type in