Moderators of highly restrictive instances (the most populous ones) blacklist all the instances they don't like. In fact most of those instances use a shared blacklist. So you have to make a choice: either you choose a restrictive instance and you can't talk about whatever you want (because with so many rules, where are they even drawing the line), or you go to a free instance and can't talk to anybody in the most populous instances (most of the network).

Or you simply get the best of both worlds and stay on Twitter.