Because at best Instagram is very fussy about keeping a brand-safe platform and an algorithm-driven feed that is not the sort of thing who favors smaller creators. Threads is an off-shoot of Instagram, likely it will be treated the same way.
When Twitter was at its best, people liked it for how it allowed them to keep up with current events, and to spread interesting content through viral sharing, as users with similar interests spread it among each other. I don't think Facebook will allow it to be user-driven enough for it to have this sort of organic virality.
What you should worry about is Zuck interest in the fediverse. It might boost the fediverse but he might somehow take over and become chancellor Sutler.
He wants to outdo the rest of the fediverse in features and users and then centralize bluesky and/or defederate from mastodon and others so that they can essentially attempt to kill off the fediverse.
Bluesky isn't twitter federated. Bluesky is the death of federation.