I really dont think the water table has anything to do with it seeing as Space Mountain is sunk 17feet below the MK and the entire MK is a full 1 (or is it 2?) stories above ground anyway.
Its just that the MK is bigger so they were able to sit the show building within the park, whereas in DL they needed to place the showbuilding outside the railroad tracks because there was no available space within the park, therefore they had to create a way for guests to cross under the railroad to get into the ride. so building the elevator in florida would have been a bit of a waste of $$