Pardon me for the tangent...
You know, it's funny. Prior to the release of Wall-E, I used to think it would be cool if Disney World had a "super" version of the People Mover. You'd get in your car when you arrive at the park, and it transports you to the rides, becomes your ride vehicle on the ride, your table at restaurants, your seat for the parades and fireworks, etc. It would be covered and air conditioned, so you'd never have to worry about rain, heat, or sunburns. Think of all the walking and standing in lines it would save you, I used to think. It would be the perfect way to visit Disney World.
Sitting in the theater watching Wall-E when it came out, my mind went immediately to this concept of the "Super People Mover" I'd been hoping for as soon as we saw the Axiom. Yep, it was pretty much what I'd envisioned. The message in the movie was a good ol' slap in the face that made me feel guilty enough that I stopped wishing for my "Super People Mover."
Back on topic...
I think the reason Wall-E hasn't been inserted into Future World is because it doesn't
exactly match the theme. Future World's theme is a querky 1950's vision of the future. Wall-E is more of a modern day vision of a future. (Of course, Stitch, Monsters Inc., and Buzz Lightyear really don't fit the 1950's vision, either. So there goes that theory.) Hmmmm... perhaps it's the vision of a future in which mankind has literally destroyed the Earth that gives them pause?
I was going to say it was the bean counters, but Wall-E actually grossed more than some of these other movies that have attractions in Future World.