In 1776, both American accents and British accents were largely rhotic.
It was around this time that non-rhotic speech took off in southern England, especially among the upper class; this “prestige” non-rhotic speech was standardized, and has been spreading in Britain ever since.
Most American accents, however, remained rhotic.
There are a few fascinating exceptions: New York and New England accents became non-rhotic, perhaps because of the region’s British connections. Irish and Scottish accents are still rhotic.
Didn't see that one coming. Very cool. And very good timing for this post, Nick Patrick! Happy birthday, USA.