My understanding is that they realized that the Mirror Fusion Test Facility wasn't actually going to work and wasn't a plausible way forward to an actually functional reactor late enough in the construction process that it made more sense to finish it and shut it down rather than just cancel it immediately. @asnys knows more about this subject, I think...With a strong force to push for Fusion, it's likely you'd see the Center for Fusion Engineering be created while projects like Princeton’s Tokamak Fusion Test Reactor (TFTR) and Lawrence Livermore Laboratory’s Mirror Fusion Test Reactor get a chance to actually operate, instead of shut down on the day they were supposed in the example of the latter.