As successful as SLS has been, it has yet to be flown into LLO, much less to Mars. Until it has many successful flights (including carrying humans) to Lunar orbit at least, there are no guarantees that it can do the job. What if after all the money and effort, it can’t live up to its promise? It wouldn’t be the first program that has happened to over the years. I’d love to see it succeed, but it might not. We just don’t know yet.
Whether we like it or not, SpaceX does have the core capabilities in building human rated boosters and spacecraft.