It's pretty easy to understand as far as the States go. It's a nation of nations, populated by people from the world over. The regions with the most contact with the rest of the world, primarily the northeast and the west coast, have accents best described as an amalgamation of everyone who's ever passed through or lived there. As these people pushed west and east, they became separated from everyone else, retaining their accents while everyone else's changed as a result of a kind of linguistic evolution. Then you have places like the UK, where the upper class went out of its way to change the way they speak to create a barrier between them and the lower class, only to have the lower classes mimic that accent creating what we hear today. A closer approximation of how they spoke in the 1700s would probably be the accent you hear in Virginia. The Spanish did the same thing, except they decided to add lisps to everything. And now the rest of the spanish-speaking world makes fun of them.