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.