A little late to join the party, but the theme is interesting...
Sorry for my bad english, there are several years I do not write in english so much.
in reality, Italy pionereed the use aerial auto cannons, with the 25mm Fiat-Revelli gun (magazine fed, eight round), about 200 of which were installed, during WWI, in the Macchi flying boats. The weapon proved to be reliable, and light enough to be installed in flexible mountings, but was unfavoured by the crews cause, using HE rounds, if, for every reason, the action would cycle without having correctly extracted the last case, the result would be deadly for the gunner (later, both the tray-fed Breda 20/65 and Scotti 20/77 had an internal safety to prevent that).
An eventual development, done by replacing the big and slow 25mm projectile with a smaller and faster 20mm one, in the same 87mm case, and feeding it with a drum magazine, would have resulted in a rough equivalent of the MGFF. But in the '30s Fiat was no more producing firearms, and probably almost nobody remembered the weapon.
There are several reasons to the italian failure of developing a 20mm aerial cannon prior to the war, but, the first one, was surely the time factor.
That's difficult to understand today, when 20 years old fighter are still good for first-line service, but, in a 5 years span only, in the second half of the '30s (moreover, a period of economic crisis), aero fighters developed from biplanes armed with a pair of rifle-calibre MGs, to metallic monoplanes armed with eight MGs, three cannons, and so on.
In this scenery, the nations that recognized first than the others the necessity of improving the firepower of their planes, risked to be outgunned by the last ditch efforts made by the others, literally few days before the outbreak of the war.
That's the present case. Italy was one of the first powers in recognising the necessity of replacing the rifle-calibre MGs with heavier weapons. The studies for the 12.7mm Breda-SAFAT begun at the end of '20s, the weapon was ready in the early '30s, experimentally installed in some Fiat CR30, and standard in Fiat CR32 (first flew in 1933, in line in 1935).
Equipped with them, for example, the pilots of CR.32s (a fighter they already know would soon be replaced, and it was only two years old) that witnessed the brand new Bf109 B1, B2 and C in action in Spain 1937-38, could hardly have been impressed by their armament of two, three (whit the often-jammed third MG of the Bf109b2 making bad press to propeller-hub-firing weapons also) and four rifle-calibre MGs respectively (and also by their 640hp Junkers Jumo 210 engine for that matter).
In the end, still in 1938, a pilot armed with a pair of heavy machine guns, could think to be very well armed compared to its opponents (and the competition to choose the monoplane fighter of the Regia was launched in 1937). A year later, with the same armament (and a much more powerful engine) he was outgunned (and outengined).
We can see the same problem whit the other power that choose to rely on heavy MGs.
In 1936 the Grumman F3F entered in line armed wit a 0.30 and a 0.50 Browning. In 1937 the P-35 had two 0.30 and two 0.50. So was armed the P-40 until the D Version (and, at that point, the war in Europe had already begun). Better than the Breda SAFAT couple, but not an impressive armament at all. But the U.S. entered the war later, and had the time, and the manufacturing capability, to use the European lesson.
The second part of the equation is the different role of the weapon manufacturers, the aircraft manufacturers, and the government.
The Breda firm helped producing the Fiat-Revelli MG during WWI, but, in the mid '20s, decided to enter in the MG market with autonomous projects, in competition with the Fiat ones. It's first products were a series of recoil operated weapon, using the patented Mascarucci breechlock (ironically, the same development led to the highly humoral Breda-30 and the stone-axe reliable Breda-SAFAT) whose ultimate development, the will-be Breda SAFAT, won the competition with the Fiat concurrent product and the subsequent lawsuit brought by Fiat itself. As a result Fiat decided to leave the firearms market, selling his specialized subsidiary SAFAT to the Breda itself.
At that point Breda was pratically a monopolist in the Italian heavy MG market, since the only Italian competitor, the "Autoscotti" was not really a manufacturer, but a single inventor, Alfredo Scotti, who sold the projects of the weapons he designed, (all, from the "Scotti model X" rifle, to the "Cannone Mitragliera 20/70", based on the same gas-unlocked-blowback mechanism he patented in 1928 ), to other firms, as the Swiss Oerlikon, or the Isotta Fraschini.
But, this did not slow the inventiveness of Breda technicians. In 1929 the firm acquired the licence to produce the gas operated Hotchkiss 13.2mm MG (after two years of development, that became the Breda-31, adopted by the navy), and then, having developed a gas operated mechanism of his own (single lug, rising bolt due to the inclined surfaces in the bolt carrier), produced a series of gas operated weapons (6.5mm-8mm Breda-PG battle rifle, 8mm Breda-37 and Breda-38 MGs, 20mm Breda-20/65 and 37mm Breda-37/54, all introduced from 1934 to 1938, quite a result for a brand that had first experienced gas operated MGs only on the Hotchkiss copy) which all turned out to be sturdy and reliable, and were much appreciated by users. However all those, as the Scotti MGs, fired from open bolt, so were apt for long bursts without overheating, but were not synchronizable.
In the meantime, the Breda SAFAT, having won his competition, remained at the level of evolution it had in the early '30s (good for those years, but no more at the end of the decade) without attempts to adapt it to a more powerful cartridge (as the 13.2X96 used on Breda-31) or to lighten it further.
It must be said, however, that all the Breda and Scotti MGs, with the exception of the Breda-PG, were developed at the specific request of the government, which paid for the prototypes. Both Breda and Scotti demonstrated not having problems in scaling up and down their models. If there was a request of the government for a lighter aircraft cannon, they would certainly have been able to lighten their 20mm autocannons to fire a less potent shell than the 20 X 138b Long Solothurn (for example, the 20 X 105b Short Solothurn, that, firing the same bullet, would have had some manufacturing advantage). But there never was such a request.
The Navy was able to understand that the heavy MG they adopted in '31 for AA defence was rapidly becoming obsolete, and began to replace them with the 20mm Breda 20/65 in 1935. The Air Force did not ask for a 20mm gun adapted to his needs, not even to give to aircraft designers an alternative option to the heavy MGs.
In fact, The competition for monoplane fighter of 1937 nominally required, as a minimum armament, two heavy MGs, or two cannons. That for an assault aircraft, of the same year, nominally required 4 MG, or cannons. But in reality there was no alternative for producers. If they had really wanted to install the cannons, they would have had to adress to foreign products (with the Oerlikon products being the only real possibility, as Italy was under international sanctions), something that certainly did not make the plane more attractive for the decisors, without even being able to really obtain them. In fact, in the prototype of the Ambrosini SS4, where a cannon (most likely a MGFF) was projected, no cannon was mounted. The supply of weapons, as that of the engines, was in charge of the Air Force, and only if the prototype had been accepted, it would have started the procedures to import the guns from foreign manufacturers.
It's worth to say however that, in later '30s, existing 20mm aerial cannons were not so attractive, being, with few exceptions, drum fed and slow firing, so forcing the aircraft to still have at least a couple of supporting MGs, for when the gun end the shots, and for deflection shooting.