A guess would be that fantasy worlds are often based on mediaeval England and the Europeans only settled North America after the mediaeval period and therefore an American accent seems to be anachronistic.
Bad guys are British because historically Britain was the overbearing exploitive world superpower that the colonies managed to oust. I believe that the relations between the U.S and Britain were rocky right up until second world war. The bad guys always speak with plummy RP, seldom a regional accent. If I remember correctly, a bad officer in Titanic spoke posh, while a good officer had a Welsh accent.
