I think the starting points were different for the US and UK. Here in the UK we had many aircraft fitted with light machine guns effectively firing a rifle bullet. The performance of this small calibre was marginal and we took a fresh look and migrated towards cannons.
The US started with a .50 cal HMG round which provided them with a superior capability from the outset. The US recognised the advantages of the cannon but the capability increase from the .50 cal was not as urgently needed compared to those who pitched-up to a war firing a bullet around a 1/4 of the weight and half the effective range of the .50 cal.
(NB some US aircraft had .30 cal weapons fitted and $ price comes into everything)
