Well, it makes sense from their standpoint. They want to have a "base" world that isn't developed very much so it doesn't feel like people need to buy supplements for that world or they won't otherwise be playing "real" D&D. Greyhawk has been "frozen" with no new products coming out, so it requires nothing world-specific for it.
Forgotten Realms, on the other hand, demands more than that. It has its own campaign setting, monster book, DM's screen, etc. If it were to be the default world in the Core Rulebooks, the pressure would be greater to go out and get those additional books.