Because they had to. Potter had lost his mojo and they needed a stop gap between Potter 1 and 2. Potter 2 should resolve the problem for good. Although they are bringing in Kong just in case.
IMO.
PS- 7DMT was slow due to being surrounded by a working theme park. No doubt about that. Also it is new ride tech. Just learned Shanghai is getting a clone so you can be sure Disney went carefully to make sure this went as smoothly as possible because you know there is a lot of incentive to have shanghai go as smoothly as possible.
JT, those are the worst excuses. How can you ignore that Transformers was IN a working theme park as well. The new tech for 7dmt is basically swinging cars and most have said they don't swing that much anyway so why would that slow construction down? I doubt getting it right for Shanghai had anything to do it as well. It's OK to admit Disney just took their sweet as time and wanted to make sure it was timed with HP opening because they have little else to combat that.
HP opening will no doubt have people booking trips to Orlando and those folks will even visit Disney during their trip. UNI and HP will be bringing more money for Disney than anything WDW has built recently, 7DMT included.