The reason for the closure was indeed for the lack of wheelchair accommodation. If there were a fire in the building, disabling the elevator, there would be no way for guests in wheelchairs to get out of the building without being carried down stairs or escalators.
The reason the Land can get away with this is because their are 2 ways out from that building. The first way is out through the 2nd floor where traffic normally flows in and out. The 2nd way is through the emergency exit doors next to Soarin, which will lead out next to the Imagination pavilion. With this set up, if the elevator were to not work during an evacuation, all wheelchairs would be able to exit the building.
Reedy Creek took a look at Imag's 2nd floor and said "if you want guests up here, you need an alternate exit for guests in wheelchairs", so that's where it stopped.