That's a US cinema tradition going back to post-WWII epics like Quo Vadis and Spartacus, in which the heroes are played by Manly [American] Men like Kirk Douglas or Robert Taylor (who BTW was widely rumo[u]red™ to be gay] , while the effete villains - think Peter Ustinov's Nero or Olivier's Crassus - always sport an English accent.

By way of compensation, though, the innocent Christian heroines were Jean Simmons, Deborah Kerr, etc.

And saints always sounded like Midwestern preachers.