Why did they even bother?

Same reason they made 42, despite the fact that there was already The Jackie Robinson Story, starring Jackie Robinson. Because for all his natural charisma, Ali was not an actor, and that movie - as a movie - is not very good. Not to mention, Mario van Peebles is a much more convincing Malcolm X than James Earl Jones. I mean, I love James Earl Jones...but he looks about as much like Malcolm as Marlon Brando looked like Henry Fonda.