MN used to be self-moderated.
What happened was that posters ended up suffering from what I call "arsehole fatigue". That is, some complete wanker/troll/GF would come on and post really hateful, offensive crap and it would be left to MNers to argue with them/point out the error of their ways.
It was exhausting and in no way commensurate.
The GF/Troll had their lolz but the posters left to point out that posting hateful comments about race/disability/etc was unreasonable were often left quite upset about the experience (I suspect that's part of the LOLZ for the GF).
There is also the argument that you change the nature of public space (be that an internet forum of the set of protocols determining which kinds of speech are and are not tolerable within a given polis at a given time) if you manage to just generate a vast quantity of quite hateful speech in a public communicative arena.
There is also the argument that an inclusive open space cannot be filled with hate speech against certain groups because - rather than the non-patrolled public space being neutral through a non-governing of speech in that public space - it actually acts to keep people out of the public space. Who would feel welcome in a public forum that routinely permits anti-semitic speech? Who would be given the message that the forum is for anti-semites primarily?
In the old, self-modded days, MN was something of a target for those who got their jollies by posting anti-SEND speech. It did not make for a pleasant experience for the people actually arguing against.
I think there's also a gender element to this: 'arguing against' on a site such as MN, is women's work, and so the labour - emotional - is invisible. And it's women who tend to the caring for children with SEND, so there is something incredibly anti-woman about it. Thinking that is a 'nothing' cost versus the 'right' to some (fairly unanalysed) notion of 'free speech' renders women's work invisible.
So MNHQ started mod-ing the site.