When executing a plan as little variables as possible. If he gets censored its not like he will have millions of followers the next day on whatever platform he moves to.
Also, if he moves now its Trump supports Nazi platform, they will still do that anyways, but he has the cover of the talking heads saying he wouldn't have to move if Twitter didn't ban political speech he didn't like and that is the topic instead.
And also reach. Twitter is where its at right now.