OnPeak (the vendor Celebration uses) is allotted a limited block of rooms, and it's those rooms that are sold out.
If you are a member of a chain's rewards program, you may be able to book a room at one of those same hotels for a comparable rate. For instance, I'm a member of Hilton's rewards program, and I got a similar rate at the Hilton that the link through the Celebration webpage.
I also got a lower rate at the Days Inn as a federal government employee (yes, I'm that guy who sits on hotel reservations on rooms that have generous cancellation policies; I like options).
You might also call the hotel directly and make a reservation through the hotel's staff (who may also get you a better rate than the website).
Another nice thing about Orlando is that there are a gazillion rooms in the area. So even if you end up having to drive a little ways from International Drive, US-192 or even the Disney property, you can probably get a room at a not-too inflated rate (Celebration is also during Spring Break season, so there will be a little bit of a bump in price).
