Because Medusa's red hair is WAYY too iconic and I'm not sure how it would look on a nonwhite lady (comic drawing or actress) This isn't a MJ type situation, her hair is literally her superpower and it's supposed to look like a massive river of red. As for Crystal, well she is her sister, not an adopted one, and they would need to look like they come from the same family.
My dream fancast for Medusa is Rebecca Ferguson after seeing her in Dune, as for Black Bolt - ever since the rumor about Farhan Akhtar being in Ms. Marvel TV series I can't help but fancast him as perfect for that role. I haven't even heard of the guy before that rumor came out and now I've seen several of his movies lol.