Can't speak for anyone else, but the main reason I stopped playing on UMcS a couple years ago now were a combination of one person who shouldn't have had mod powers using them to threaten me after I killed him in a traitor round (despite having given him every possible warning to FUCK OFF I'M DOING TATOR STUFF short of telling him) and also frequently playing shitcurity/Cap-as-shitcurity on lowpop (including things like being HoS on a round with like 6 people, beating me down and 10min brigging me because I was breaking into medical supply as chemist while being literally the only person in medical to ensure people would have stuff to heal themselves while I was prepping pills, since I'd naturally used all of my initial energy on making drugs). That and a certain other person occasionally using admeme powers to force-antag themselves and grief.
Haven't played since pop picked back up so I don't know if that sort of thing is still an issue, but there's also that my spare time is sufficiently limited that I'd rather play CM instead of spending 4 hours doing nothing on extended or an antag round with so little antagging that it's functionally extended anyways. You can only play slime rancher or whatever so many times before the process itself becomes boring.