Hiya, this question has come up in this sub a few times, but I see that none of the old answers are up to snuff. Hopefully this time we'll get some expert responses, but in the meantime, I'd suggest that the reason is that it's not really a history question (as in, people in Ancient Rome did not speak with modern British accents) as much as it is a dramatic convention. Maybe x-post to a sub that specializes in dramatic criticism, and/or /r/AskSocialScience for connotations of British - and especially "upper class" British - accents.