Rachel provides two key reasons (my phrasing):
1. #Threads wants to be the best place where people have relevant, public, real-time conversations, and the #fediverse is the means by which people can find the audiences that are best for those conversations, because not all of them will be on Threads.
2. The fediverse is a compelling way for creators to own their audiences in a way they aren't able to own on other apps today.