The short version is because the mods there are very special (in abad way) and like to gatekeep their echochamber as much as possible.
The long one is because they have a vision where their platform is more safe to its users which should be more likeminded individuals and would like to keep it as true and safe as possible acording to their vision, which im not sure what it really is, from what i heard is that they are very left leaning but in an extreme way and dont like discidents, so anyone thats not as likeminded as them is just straight up not welcome then. They probably call disicidents harasers and just ban them. So an echochamber.
Now officially what they said is that everybody but them sucks at moderating and wont refederate untill they dont suck anymore. They say thats because everyone doesnt have sophisticated tools for moderating but i think they just whant everyone to enforce their ideals everywhere else, and if thats the case i believe we are better of without them.