Sponson:
The US could not make a turret large enough for a 75 MM gun so that gun was installed on the side of the take, the sponson.
Sponson:
Later, as we mastered the technology to make a turret capable of mounting a larger gun, the Army made the unfortunate decision to stick with the 75mm as the main gun for the M-4 Sherman. Initially, the 75 was adequate against the older German tanks we encountered in North Africa, but as the Panther entered wider service, it became obvious that our Shermans were out-gunned by the 88mm on German Panther and Tiger tanks.
The reason we chose the 75mm? The Army artillery branch pushed for it because it was effective as an infantry support weapon and we had a large stockpile of 75mm HE shells. Also worth remembering that the Army’s entire R&D budget for tank development in 1940 was less than $90,000.
A lot of American tank crews paid with their lives for our parsimony in the pre-war years.