Many reasons. Perhaps you don't know about the occupancy levels and the room night demand for WDW. At this point, building more value room nights into their inventory would not increase their revenue enough to support the cost.
Couple that with the internal demands, taxes, staffing and the rest, and Disney has made a business choice not to go ahead with it..
Also that site has to be torn down and built back up again from scratch...
Pretty clear what they are doing...............