So it really boils down to two major points:
1. Logistics/commonality. It was easy to mass produce large numbers of .50 BMG quickly, and provide lots of ammo for those guns which were common to lots of other vehicles, to include a dozen or more on every B-17 and B-24 bomber, plus the numerous guns on B-20, B-25, B-26, etc etc., And it was easy to get all those guns and all that ammo from the USA to the far end of the Atlantic and the Pacific. This was essentially the same reason that US tanks used gasoline engines rather than diesel. Despite the far greater fire danger posed by gasoline, it simplified logistics by having a single type of fuel for everything from light jeeps to heavy tanks.
2. Good enough. The targets being shot at were light enough that a .50 BMG did the job. Neither the Germans nor the Japanese had large bombers that needed to be shot down. The exceptions of cannons on US aircraft was for air to ground use, not air to air. And even then, large numbers of .50 BMG rounds on a truck or other ground vehicle (even armored ones) were effective at disabling them even if they did not destroy them. And often disabling was all that was needed. So the crew flying them usually preferred the machine gun equipped aircraft over the cannon equipped aircraft.
There is a third lesser point. Both the AN/M2 (the lightweight aviation version of the ubiquitous "Ma Deuce") and its ammo was relatively small and compared to cannon, very light. Both are very important considerations in a fighter.
Last edited by KenV; 1st Mar 2018 at 14:22.