Because if you're gonna make a Superman movie with a black Superman, you should change his backstory to be culturally relevant and he would have inevitably faced racism. A black guy in a small town, living on a farm with a traditional upbringing is silly and a missed opportunity. Standing up for truth, justice and the American way having been a victim of racism? Working with the institutionally racist police and military? You're just erasing all that by just making Superman black.
It's the same with James Bond to be honest. Or Batman. I don't really have faith they could logically do it anyway. I didn't think Black Panther was a good movie, but it did all these aspects well.