Britain has had the same population and culture for hundreds of years. Americans have a strong British heritage, but even their early population includes Germans (Hessians), Dutch, French, Jews and blacks from Africa. This varied mixture of population and languages altered the American accent from that of the original British.
I can only second that as a correct answer. Americans speak how English people used to speak centuries ago. That doesn't stop the modern Englishmen to cuss Americans though for not speaking the "correct" English... add a comment