You're implying these other black characters are somehow better off in these regards and they're not. At least we actually know there are more plans for Naomi stories, while Vixen and John Stewart more or less don't have anything going on for them. Vixen, John Stewart, and so on may have been around a long time, but that's not what matters. What's important is what they've done in the time they've been around. And they haven't done a whole lot for as long as they've existed. Especially Vixen. What's more is that no one is likely to do anything for them because they have very little incentive to. You couldn't even pay little old me enough to spend my time toiling building up John Stewart or Vixen, and giving them my best ideas. I'd use that effort on my own stuff. A lot of comics creators feel similarly, which is why they don't often create a whole lot for characters like these. On the other hand, I'm pretty confident Bendis will give his best efforts building Naomi up in the stories to come.