There isn't enough space in Epcot? It's the largest park!
As far as convention space goes, there is:
-Wonders of Life/Festival Center pavilion
-Millennium Village building
-Odyssey building
-Convention pads near the bridge between UK and France as well as in Italy
Compare that alone to the other parks.
Disney's Hollywood Studios uses Streets of America & the end of Sunset Blvd for conventions. They also use LMA, Indy, a building near the Backlot Tour, and sometimes The Great Movie Ride.
Magic Kingdom doesn't have convention and special event space, because it is the most expensive park to book, but I think they use Tomorrowland Terrace as one area and Hall of Presidents as another.
I'm not sure about Animal Kingdom.
Epcot doesn't need to use all those facilities solely as special event space. They have other options as well. Why can't the Oddessy building serve as a the Festival Center. It is more centrally located after all and closer to Food & Wine and FLower & Garden stuff.