Disney has booked the Anaheim Convention Center every year in early September through 2012.
If they held a D23 Expo on WDW property in 2013 it would be a much smaller, simpler version of it with a lot missing from the Anaheim version.
D23 Expo at WDW would have to be split up between five different conference facilities spread all over WDW property, and the vibe and energy would be very, very different because of that. You would also be missing a venue like the 7,000 seat Anaheim Arena at WDW, and that's where they held the major executive presentations.
D23 Expo in 2009 used 550,000 square feet of Anaheim Convention Center exhibit halls, theaters, and Arena space. There is still a 225,000 square feet exhibit hall at the Anaheim Convention Center that wasn't used in 2009 that could be used for expansion in future years, plus about 150,000 square feet of additional presentation rooms and ballrooms to be used.
All of the WDW conference facilities combined and maxed out completely at the Dolphin, Coronado Springs, Contemporary, Boardwalk, Grand Floridian, Beach Club equal just 525,000 square feet with no expansion available and no indoor arena available to seat 5,000+ visitors.
Plus the logistics and prohibitively huge cost of transporting all of the exhibits and personnel across crountry ensure that if D23 Expo is ever held in Orlando instead of Anaheim it will be a very, very different type of experience. Not only is the Anaheim Convention Center a short stroll from Disneyland, it's a 45 minute freeway drive from Burbank and Glendale, and a 60 minute drive from Hollywood. Disney can get just about anyone to Anaheim very easy.
D23 Expo will remain in Anaheim for many, many reasons.