One of Greyhawk's main problem is that it failed to achieve any significant traction beyond a core group of fans back in the early 80's. Yeah, I'm sure there are people who came to Greyhawk later, but not enough to really make a difference. And as I've said before, that original core group of fans can be hard to please and TSR managed to damage the brand, perhaps irreparably, right after Gary's departure. From the Ashes had a decidedly mixed reception as did WotC's attempt to revive the setting. Erik Mona's work was better received, but it probably helped that he's a Greyhawk fan himself and understands what the fanbase likes.
It doesn't really help that Greyhawk and the Realms are both very similar. The differences tend to heavily overstated. To me the biggest difference is that Greyhawk was designed to be used in an early wargaming/roleplaying hybrid campaign while the Realms seems to be strongly influenced by the game's literary influences. Even then, these are still minor differences really. I've been playing nearly 30 years, and I've never used either setting for a campaign. I can tell the difference between the two, but even then they're still both vanilla settings to me. To a new player, there isn't really much to distinguish the two. It's kind of a shame because I think Greyhawk's overall flavor is a little better than the Realms.