If it’s a film/show about his life, a black man leading the civil rights movement, of course you need a black man to play him. Whenever white historical figures have had their stories told through films they’re always played by white actors because often they’re made to look like the real person. However, if it’s a made up tv show it doesn’t really matter if a poc is playing some rich aristocrat, even if it’s highly unlikely that would actually happen, because it’s not real. Same with musicals, I know SIX is based on historical events but it’s a modern retelling, so it doesn’t matter that the cast isn’t completely white.