Because to the American ear, the Brittish accent sounds weird (snobby & evil). That's why in the old films, the Nazi mastermind often spoke with a very clipped 'educated' English accent (which is so hilarious, when you think about it).
There's something scarier about a bad guy who's less emotional, more intelligent, well-spoken & well-dressed, which a lot of these stereotypes are. Take for example, Gus off Breaking Bad. He's not English but I think it works from the same premise.