I have to admit, I can't get enough of this Fall's D&D event, Rage of Demons. I have a feeling a vast majority of D&D fans can't get enough either. So what is it about demons and demon lords that captures the imaginations of RPG fans (as much as dragons)? It's probably a long and complicated answer, but I can provide a couple concrete reasons to build upon: demons are an unrepentant evil and they look damn cool. Dragons look cool too, but being evil isn't necessarily their whole reason for existence. There can be exceptions. Not so with D&D demon-kind (or Tanar'ri as they were known in the dark times). Demon lords especially are totally, 100% irredeemable in this game so that makes them the best villains. These aren't demons like you'll see on Buffy (except maybe Graz'zt). That means there is no ambiguity or confusion (unless they create it) in defeating them since they certainly won't feel bad taking your character's soul first!
For these reasons Greyhawk has always been a hot-bed of demonic plots over the years. From the very beginning, Iuz the half-demon son of the demon lord Graz'zt has been the primary evil threatening the Flanaess. And his witch of a mother, Iggwilv is the main authority on demonology with her famous Demonomicon. Then nearby, trapped in the Temple of Elemental Evil lurks Iuz's ally the queen of fungi Zuggtmoy. Beneath the mountains, Lolth causes trouble for Oerth all the way from her Demonweb. That's just for starters.
Fraz-Urb'luu was once trapped beneath Greyhawk Ruins by mad Zagyg, the Prince of Demons Demogorgon tried to wreck the world with his Savage Tide, the Queen of Chaos terrorized Oerth in prehistory against the Rod of Seven Parts, and the foul Kerzit menaced the dungeons of Maure Castle. There was simply so many demons in Greyhawk at one point that the Crook of Rao had to be used to chase off most of them.
So have fun playing Out of Abyss as they tear-up the Forgotten Realms. Or maybe an enterprising DM can adapt the module to Greyhawk. It's not like Oerth can't handle a few pesky Demon Lords.