Why do they keep changing Protoss mechanics

First we had the mothership with vortex, then vortex was gone. Then mothership core was a thing, then it wasn't. Then the nexus could shoot, then it couldn't. Then the shield battery could be overcharged, now it can't.

I feel like terran and zerg mechanics have been pretty stable but protoss they change all the time. Why can't they get protoss "right"?