It's funny, you can see the reasoning in having a UK-only cast in some film or series actually set somewhere in Britain in the remote past - notwithstanding the fact that accents and pronunciation have changed a lot, and that Shakespeare probably sounded like a cross between a modern Brummie and someone from one of those weird inbred communities in a remote coastal part of West Virginia. But there's seemingly a complete ban on American accents, not only in productions set in the real historical past, but even those set in a fantasy world like Westeros or Middle-earth.