It's not just the absence of an a academy resisting linguistic change, it's that the people who brought English to North America were English, so the dialects t

It's not just the absence of an a academy resisting linguistic change, it's that the people who brought English to North America were English, so the dialects that have evolved in North America have been evolving from Old English just as long as the dialects in England. And moreover, languages don't change at a uniform rate. Until recently the center of linguistic change in the anglosphere was London, so British English, and particularly London English, was more innovative than North American dialects. I expect things have gotten more complicated since there are populous power centers outside of London now.