The short answer is that the different American
accents are partly due to settlers from different parts of
England (yes, and the rest of the world). The
British Isles has a huge range of very distinct, very strong accents (even
dialects in some parts).
So you have a bit of
London mixed in with a bit of
East Anglia, mixed in with some
West country, etc and scattered across the US.
But, these accents were
exported several hundred years ago. So, the sounds on both sides of the ocean have
diverged, and the accents are no longer a neat match.
That said, there is a remote fishing community somewhere on the North Eastern coast that has an accent that is understood to be very close to "
Shakespeare's" English (that is, from around
Stratford in the late sixteenth, early seventeenth century).