Please bear in mind that I'm a lemmy.ml user (though not a tankie, nor marxist, nor even socialist.)
Look at OP. How many lines are about smearing someone for their politics and beliefs (Even the subject line covers ONLY that), and how many are about the need to improve moderation practices at .ml (precious few, not even the subject line).
IMO that's one reason. The discussion rarely ends up being about moderation practices even when that's the stated goal. It ends up being about not liking someone's views.
The other reasons - you can personally block that instance if you choose, but for it to be hidden from EVERYONE on an instance, the admins of the instances need to make that decision. (To defederate.) There are lots of potential reasons not to do so, and I think many of them boil down to not throwing out the baby with the bathwater since there are plenty of users on ml like me who are not tankies and are just having the same discussions we'd have anywhere.
I got a 3 day ban from one single community at .ml within my first couple days here for using the term "whataboutism" - I suspect it was an automod action. I wasn't really happy about it, but eh? shrug
I have the same answer for folks now that I did when social media was somehow full of US conservatives claiming they could no longer speak their minds on social media. No one is obligated to give me or you or anyone a platform. If Lemmy.ml is that much of a shithole, it will eventually get defederated broadly, and everyone who isn't a tankie will stop using it.
The occasional thread like this, or someone complaining that my opinion is invalid because I'm an ml user (has happened maybe three times) are the only times I ever think about it really.