Adam Mosseri:
Second, threads posted by me and a few members of the Threads team will be available on other fediverse platforms like Mastodon starting this week. This test is a small but meaningful step towards making Threads interoperable with other apps using ActivityPub — we’re committed to doing this so that people can find community and engage with the content most relevant to them, no matter what app they use.
Same reason why Telegram friends won’t go to Signal: they don’t care about the platform they use, and you end up being that friend if you ask them to change their habits for you.
Once Threads support federation in both direction, the need to move disappears completely. Why would you move to a server run by volunteers that sometimes goes down when Elon says something stupid, especially if your Mastodon friends can interact with your account like normal. That’s ActivityPub working and doing what it’s supposed to do.