I think you’ve nailed it here, though perhaps a bit cynical. I personally wanted to see scientists and journalists, but few were on mastodon at the time. It’s a tiny bit better now but still not good.

Compounding problems is mastodons hate for indexing and search. This made it nearly impossible to actually find the few people I would have been interested in on mastodon.

The decentralization and poor UX I think was also a big factor for many. Apps like Ivory have helped the UX a lot but it’s still not as easy to use and discover new people as twitter.