"DC could've started it. As I mentioned in the latest Green Lantern story, Superman was all set to do a cameo. There hasn't been a reason why he's not now. We can only assume it's that legal crap that's been going on. "
More likely it's that the Superman reboot that was in the works fell apart.
It's all a little complicated. For example, the Milestone heroes only lasted a few years in print, but Static did well enough on television that DC ended up renting the rights to the characters and bring him back. Same goes for Blade and now possibly the Human Target.