.50 makes A LOT of sense in terms of supply also. It was widely used on by infantry, cavalry, tanks, boats, ships, submarines, light fighters, heavy fighters, bombers. It was literally EVERYWHERE. If the supply chain completely broke down, fighters would be able to use any of it, even if belts didn't have the higher tracer / API count used on aircraft.
The later 30mms didn't have an advantage in terms of time to target, like 50 didn't count unless they hit, were heavier so planes carried less, although they did more damage 50 is enough to wreck anything in an aluminum shell, but most importantly, nothing else used its ammo.