Haven’t asked the admins, but here’s my guess.
Being the most well known instance means that you get a lot of traffic. Let’s say that 1% of the people using your instance are annoying morons. In a smaller instance that 1% translates to maybe one ban a month or whatever. In a bigger place, it becomes a constant onslaught stupid idiot bullshit nobody has the time or energy to deal with. Either you get a bunch of admins and mods to deal with that nonsense or you start banning users more aggressively.
However, some part of that drama is public on https://gui.fediseer.com/instances/detail/lemmy.world. Have a look. Just scroll down to censures given and read the reasons why lemmy.world has defederated from so many instances.
Alcoholicorn@mander.xyz 2 weeks ago
To maintain an overton window firmly within liberalism. Pre-emptively defederating hexbear “as a last resort” gave the game away.
GrammarPolice@lemmy.world 2 weeks ago
If this was actually the case, they would’ve been long defederated from ml. That is yet to happen as of now