Well, i think it has to do with all the main comic icons being established back in the 40's/50's. Black people weren't as accepted back then, only you're basic caucasians were. And, in my opinion when it comes to DC and Marvel, the majority of readers don't want to see anyone take the spotlight away from their old faves and they're afraid of too much change.
Take the X-Men for example. The main X-Men are pretty much the same as they were 20 years ago. How many kids from New Mutants or Gen X made it to the big leagues. A couple yeah, but how long did they manage to stay on a team? Not long at all. Which sucks, cause we're missing out on a lot of potential.
Another example is Sentry. Sure, he's a Superman clone, but he's got alot of potential to be his own character. yet a lot of people condemn him before knowing anything about him. Mainly cause he's being marketed as Marvels most powerful hero. people don't like that he's taking spotlight from Thor, or Ironman or whoever.
That's just the way it seems to go with comics... IMO.