Pondering this, it strikes me that an interesting way to figure it out sideways is to look at what types of speech, language and class went into the mix that formed such distinct regional accents. Some different regions were settled by similar mixes of ethnic heritage, but perhaps the socio-economic class was the dominant seasoning for a particular way of talking. I know that PA Deutchisms sneak into my way of talking, and I'm one generation away from the family living on the Philly mainline (and vividly remember an Ohio nun taunting my first grade self "you mean your ANT?" when I'd said "AUNT"- accused of putting on airs)

Society and culture surely strain the mix - why does a black midwesterner sound much like a black southerner when a white midwesterner sounds very different from a white southerner?

When were the regional accents in English,first a noticeable thing on this continent? How old is a Yankee accent? When could you first tell a Canadian easily from an American by how they talk? You could do years of linguistic study on this question! What fun!

Joanne in Cleveland (which is in the western frontier of the east, and the eastern frontier of the midwest)