1. Seems some people have decided that the ActivityPub protocol was created only for the purpose of connecting non-corporate social media together. I never saw evidence that this was a design principle, but they believe it.
2. Fear that advertising will somehow come into the Fediverse.
3. Unwillingness to connect to the people who use Threads because they associate with billionaires and corporations by using their software. Their kind just isn't welcome here.
4. Meta will come for our servers, steal personal information, overwhelm our servers and leave us with a pile of burning hardware. mastodon.social, so far, seems to be doing fine.
BTW, everything in the Fediverse is in clear text, and you don't need to follow people to read their stuff. No doubt Facebook, and other corporates, and governments, have been reading everything already.
Generally, they feel in their heart that supporting Threads is supporting evil. That people who use Threads are therefore supporting evil. I've gotten plenty of nasty responses because I don't block Threads on my server. There's even a list of servers that refuse to block Threads so those fighting the evil can defederate from Threads-connected servers.
You'll find a lot of anger from some when suggesting that people on Threads be followed on the Fediverse.