often the default answer to this question is something like, "actually, the British accent changed, not the American"... Well, that's half true. The British accentS changed, especially during the Victorian Era, the second main split between the British and American dialects, as Victorian Britain turned again to France for some linguistic influence. America did no such thing, and had just released Webster's American Dictionary of the English Language, which would solidify some of the differences between BE and AmE (the -ize, -ise suffixes, etc.)
The overlying answer to your question, though, is a bit of a cop-out; as in the rest of the world, accents divulge and develop over time when isolated from one another. The modern American accents seem to be closer to the colonial accent than the modern British accents do, which leads me to my final point, relating to how the regional American accents developed, as you asked: Why did the British accent change more than the American one?
Well the regional accents of America owe their distinction simply to the normal forces of isolation which shape accents all over the world, and the linguistic heritage which gave rise to them. So for example, Boston has an usual accent partly because it has been a distinct community from outsiders to at least a small degree, and partly because early Boston settlers comprised an usually high proportion of Irish.
Regarding why Britain's accent "changed" from the colonial era... Besides my point about Victorian influence, another contributing factor is that the early colonists "average accent", if there be such a thing, was a blend of southern-English dialects. These have changed from the colonial era mainly due to the encroachment of urban accents upon the original accents (think London, growing larger throughout the last few centuries, whereas New York City hasn't spread quite so much).
Saturday, January 18, 2014
"How and when did Americans lose their British(UK) accents? How did it evolve and diverge?"