Well, even a 20mm cannon is limited in its usefullness for ground straffing. It was better than .50cal but still didn't have the power for knocking out a Panther or Tiger. This US didn't show much interest in developing a good medium sized cannon furing the 1930s, placing more emphasis on 37mm for air-to-air (which was just stupid). But once the war got going, the .50cals made perfect since for the US planes. They were very rarely going to be facing any aircraft too large to bring down by .50cal fire and the smaller ammo allowed a much larger ammo load to be carried. That is important for an escort fighter which may have to engage in a lengthy, running battle without the chance to disengage to rearm.