I'd just like to make the case for Hollywood on why they're casting white actors for this and other films where the original had non-white actors.
It's actually more about being able to sell the film to foreign territories than domestically.
In film financing the projected revenue from foreign box offices is what guarantees the producer a loan and other agreements (i.e. from banks or completion bond companies) more-so than the projected revenue from the domestic box office.
If it was determined by domestic, there would be more women, asians, africans, and other minorities in more important roles because trends show right now that Americans do want more representation on screen and in charge. But foreign projected box office > domestic....
The domestic box office projections depend heavily on factors such as actor performance (do americans want to see another damn Nic Cage movie or a Ryan Gosling movie?), timing (is there another similar movie coming out), topic (NWA Bio-pic is made for american audiences, Star Wars however is for the global audience), and many many many other factors I won't further with. Foreign box office is less picky in this aspect.
Because of these factors, it's hard to get a decent and accurate prediction from domestic box offices. The banks (or loan companies, or other important entities that help to get the film green-light) won't do business with anything that is remotely risky, banks demand their money back, it's not an investment (that's why there's interest), and it's not something they see as gambling (they won't give you a loan without a completion guarantee either).
The main reason why any Hollywood film casts more white people than any other races is because they need to meet those foreign box office projection figures. They can MORE likely meet them because the sales agents in foreign territories tell Hollywood that their audiences want to see white actors and actresses. If you try to sell the NWA bio-pic to Germany or Japan, they'll be less interested than another National Treasure movie with Nic Cage.
It's part of the cultural differences (NWA has a lot of american terminology and references whereas anyone years 5+ can probably follow anything going on in National Treasure) and it's also expectations. Foreign audiences expect Hollywood films to be as stereotypical as Michael Bay. Now I'm not saying foreign audiences are more racist or dumber than americans, it's more a factor that they don't want to pay a lot of money to see complicated diverse educationally and thought provoking films that speak about the essence of existence in multiculturalism and the polar-political atmosphere when they just wanted to see a damn Michael Bay film.
They can get that intellectual and artistic and diverse film the same you can but Hollywood, is and forever shall be Hollywood. It's not art-house indie, not Bollywood, not Scandinavian neo-noir thrillers, it's Hollywood.
So although it sounds simple to just say, "Why can't Hollywood just cast more diverse roles?" it's actually part of a global money-run ecosystem that really can't change until the cultures of the world change their expectations of Hollywood.