The U.S. was much more motorized, so the transportation issue was a lesser one. People were also thinking in trench warfare terms, so mobility would not be too important in great wars.
Actually, the U.S. Army was only moderately motorized when the first real post-Great War advanced carriage 75mm "divisional gun" was proposed, which, along with the general fiscal austerity of the times, meant that development was desultory at best. The initial design, the 75mm Gun M1 (M1923E1) and Carriage T2 (Carriage T1 standardized as the M1 (M1923E1) and was the Carriage M1916 with the kinks ironed out...hey nine years, who's to know?) and the 75mm Gun M1 and Carriage T3 were the original attempts, dating to May 1929. The 75mm M1 Gun used a vertical sliding breechblock instead of the venerable M1897's interrupted screw, and simplified the bearing surfaces and recoil mechanism over the older French design. Carriage T2 was a fairly simple improved split-trail design as a FA gun, but the interesting three-trail design, Carriage T3, was designed by Gladeon Barnes to be a universal AA, AT, and FA gun and when it was rejected (too heavy and complex, among other things), he immediately exhibited the characteristics he would later display as Chief of the Ordnance Technical Service during the war...he pouted, complained in the FA and Ordnance Journals that no one understood the genius of his design, and tried to do end-runs to get it adopted by the FA anyway (I exaggerate, but only a bit). Carriage T2 was modified as the T2E1, but also got rejected. By 1938, further work had led to modifications of the carriage (and minor modifications to the gun) through a T4 and T5. A final effort in 1938-1939 was the 75mm AA Gun and Carriage T6, which was a straightforward medium AA gun on a cruciform carriage similar to that used in the Flak 36/37...which also got rejected since the excellent 90mm AA Gun was near acceptance. However, it did not end there. The 75mm T6 Gun from that carriage got modified in 1940-1941 as the T7 and was employed as a Tank gun, first as the 75mm M2 in the Medium Tank M3 and then as the 75mm Gun M3 in the Medium Tanks M3 and M4.
The calibre was weak, but suitable-enough for AAA at the time (there were lots of heavy AAA based on soixante-quinze in active service at the time) and fine for suppressive fires. The good maximum elevation would help with impact angles that make direct hits into trenches more likely than with ordinary field guns.
Yes, but by 1938 the Coast Artillery had decided that it preferred the combination of the heavy 90mm gun and lighter 37mm gun. While they recognized the "advantages" of the 75mm "medium AA" proposal, they also saw its pitfalls, it was neither fish nor fowl, too heavy and too light, too low a Mv, too low a ceiling, but with a nice heavy projectile, something that might have benefited the Germans in their quest for a "medium" 5cm piece.