more likely because the films were mostly shot on location with mostly English actors in minor roles , I actually find it more to disturbing that they used american voice actors for ADR and overdubs for the imperials, it takes me out of the movie.
As for Vader having an english accent, I think that has more to do with him being a classically trained actor and anything that comes out the mans mouth has a regal sound to it, but it's a very neutral accent, neither English or overly American, maybe had they got better actors to play young Anakin it might have flowed better