I think I read it's more "hillbilly" English that sounds like Shakespeare? Like coal mining towns where words like "deer" and "bear" are two syllables. Probably a combination of that and eastern seaboard.

I only learned recently that the vowel shift and non-rhotic R's in Britain happened after the colonization of America. Americans still talk "normally" whereas the English got weird. Also why Irish accents sound closer to American than British I think. Linguistics is cool