yeah, what everyone else said.
Its not the equivalent of professional motor racing where you go for the technically best possible option regardless of cost. there was a war on and the 0.50 cal was readily avaialble and did the job good enough against fighters.
Add to this the fact that American made 20mm were unreliable in high vibration and flex wing mount positions and too large for "in wing" mounting in existing US aircraft and its clear why they stayed with 0.50 cal.
This link about pattern bore-sight gun harmonization might give you some insight into the some of the issues.
http://www.researcheratlarge.com/Air...nBoresighting/