I don't think I have seen a mention, except in passing, of one of the big reasons the US chose the M2. Production and logistics standardization. Yes, most upper echelon folks knew there were "better" solutions, but also had fears that production or logistics bottlenecks might be leaving fighting outfits with the wrong ammo. Or tools to fix, or whatever else. If everybody uses the same type of item, everybody always gets the right item. As our log tails got better organized, this was less of a real concern, but always remained as a BIG ghost in everybody's thinking. Standardization was viewed by many as a key to large production numbers. As an illustration - was the Sherman a good tank? Maybe, and certainly nowhere near as good, one for one, as many of the German tanks it met. But we could build thousands of them, and keep them in service, because of (relative) standardization. Assurance of meeting the level of performance required, at most times, trumped "best". The M2 performed well enough, often enough, to be the simplest solution. Again, it hadn't the highest RoF, or the highest projectile weight. But, it was the best compromise solution available.
And don't forget other production issues. If the US wants to use Hisso V cannons in everything, where is that huge number of weapons going to come from? License manufacture is an artform, from initial negotiations, to actual production line problem solving. A whole can of worms that many - right or wrong, better or worse - would not at that time willingly opened.
Most of what we are discussing here was also known to the production planners back then. If my goal is to present the "fightingest" weapons & force available, I would agree, mostly, with them. We get the Monday Morning Quarterback experience. They didn't have that option.