The trolls in the comment section at least hints at the fact that creating a more positive and constructive online space proved more difficult than they imagined.
I was curious, and joined the queue for the closed beta a long time ago. Never heard back. They explored something new in closed channels, decided not to go for it, backed out. I don't really think they need to justify the decision.
Running a social media is a huge effort, and there's a lot of trolls out there actively targeting Mozilla. I imagine it's just more trouble than it's worth.
The comments on the post also aren't from Mozilla.social. It's not like they would have been happy to see Mozilla as a successful actor on the Fediverse either.