@Dinkysaurus A bunch of folks have stated part of why, but there's kind of a bonus reason: Mastodon was starting to gain traction with one large and popular furry instance, then it fell apart overnight, and that burned a lot of people originally keen to being on the service.
Had that not happened, I think the story here would be different. And it might still be. But it made other options, even terrible ones, more attractive to earlier folks because they stay available. Then it snowballs.
@Dinkysaurus Also, a lot of folks have never gone through earlier iterations of having this happen. There are folks who have never experienced forums, or LiveJournal, or earlier places like Usenet groups, and have yet to experience a service dying up to now.
Some of us kind of take this for granted now, but folks who haven't yet will cling to the dying ship until they turn off the servers, like a bunch of us did with AIM, ICQ, and MSN.
It's scary af, but folks will find each other again.