Adding on to what /u/Mictlantecuhtli said (really though, do check the FAQ on this one):

At the time that the first English went to what is now America, they did not have the accent you're thinking of as "the English accent". There are actually many accents of English in the British Isles, but the main thing is that none of today's accents existed in the same form as they do today. All accents change and are constantly changing not only from one generation to the next, but also within a single lifetime of a single society. Your accent when you're 80 won't be what it was when you are/were 18.

Americans didn't lose their accents while Brits retained theirs. What actually happened is that accents changed about the same amount on both sides of the Atlantic.

There's always room for discussion, but perhaps the section American and British accents in our FAQ located in the sidebar will answer your inquiry.