Because you can't change the name of an entire movement. Even if you could, nothing prevents malicious Internet trolls from adopting the new name. Centralizing power under a single entity would do more harm than good because everyone disagrees with each other. If people somehow declared Internet Artistocrat to be the Supreme Leader of Not-GamerGate, then all proponents would have to do is attack Jim instead of the movement -- kind of the exact same way you're pinning responsibility of select few people who probably have nothing to do with anything on the thousands of people with pure intentions.
Basically, you are choosing to say "these random violent people represent the whole". It'd be like if saying Westboro Baptist Church represents all Christianity or if The Amazing Atheist represented all people with no belief. You are allowed to say that, but it doesn't make you any less willfully ignorant.