I got this one. It's apparently my lot in life...
The short answer is: Walt Disney World doesn't have enough space or the proper facilities to host a D23 Expo. But Anaheim does.
The long answer is: Walt Disney World's convention facilities are woefully inadequate compared to Anaheim, and what convention facilities WDW does have are split up and scattered across property so much that it would destroy any semblance of cohesion and energy and community.
The Anaheim Convention Center is the largest convention center on the West Coast. It's bigger than the convention centers in Los Angeles, or San Diego, or San Francisco, or Portland or Seattle. (And Anaheim just approved a major expansion to begin later this year, which will keep the facility the West Coast's largest for the long term, if not forever.)
They don't use all of the Anaheim Convention Center for D23 Expo, but they use a majority of the space across multiple levels and multiple exhibit halls, including the 10,000 seat Anaheim Arena for keynote addresses. Here's the square footage breakdown for the last two D23 Expo's at the Anaheim Convention Center, compared to all of the space available in every single convention hall and conference room at Walt Disney World...
Anaheim Arena (AKA the D23 Arena) - 10,000 seats, for D23 Expo keynote speeches
Exhibit Hall A - 145,000 Square Feet
Exhibit Hall B - 145,000 Square Feet
Half Exhibit Hall C - 75,000 Square Feet
Ticketing Lobby - 50,000 Square Feet
Second Floor D28 Theater/Exhibits - 50,000 Square Feet
Third Floor D23 Theater - 40,000 Square Feet
Total ACC Space Used for D23 Expo - 550,000 Square Feet
Total ACC Space Available - 1.1 Million Square Feet
D23 Arena at Anaheim Convention Center - Yes, that many people actually wanted to see Jay Rasulo!
Compare that to
all the space available across the five main convention and conference facilities at WDW; Coronado Springs, Contemporary Resort, Grand Floridian, Boardwalk, Beach & Yacht Club. First, none of those spaces has an arena like the Anaheim Arena that can seat up to 10,000. Here's how the total space available at these five WDW properties breaks down;
Coronado Springs Resort Convention Center - 220,000 Square Feet
Contemporary Resort Conference Center - 115,000 Square Feet
Yacht & Beach Club Conference Center - 70,000 Square Feet
Grand Floridian Conference Center - 40,000 Square Feet
Boardwalk Conference Center - 20,000 Square Feet
Total WDW Convention Space Available - 465,000 Square Feet
But that 465,000 square feet number actually looks better than it is. Much of that square footage is redundant lobby space, or smaller meeting rooms or ballrooms un-useable for a D23 Expo type mega-event. The largest exhibit hall at WDW, and the only full-scale exhibit hall really, is the Veracruz Hall at Coronado Springs, and it's only 86,000 square feet.
By comparison, the Anaheim Convention Center has 725,000 square feet of continuous industry-standard exhibit hall space, of which D23 Expo used 365,000 square feet in 2013. The larger pavilions like the WDI Carousel of Projects or the Disney Living Pavilion would not even fit in the Veracruz Hall without scaling down, and then you've just maxed out almost all of the space at Coronado Springs with just that one element of the Expo floor.
2013 D23 Expo Exhibit Hall Floorplan - With more theaters, exhibits, archives & presentations up on the second and third floors
http://www./wp-content/uploads/2011/08/D23Expo2011_MainFloor.gif
Even if you convinced the various divisions of the Disney Company to radically scale down the size and scope of their Exhibit Hall presentations, and were then left with a dramatically smaller and decidedly less impressive show, you would still be faced with the logistics nightmare of expecting all the D23 Expo visitors to bus themselves around WDW property to take in the WDI pavilion at Coronado Springs, the Disney Living pavilion at the Contemporary (whose full exhibit space is actually a ballroom of only 44,000 square feet), the Disney Archives exhibit at the Grand Floridian, 1,000 seat theater at the Yacht Club, etc., etc.
And... any event planner worth their headset would never try to stage such an event broken up over a half dozen venues, none of them within walking distance of each other. There would be no buzz, no vibe, no excitement, no sense of community and spectacle under such a fractured WDW scenario.
And one last reason that can't be overlooked... D23 Expo at the Anaheim Convention Center is
right across the street from Walt Disney's Magic Kingdom of Disneyland!
How cool is that?!?