The issue is quality. 2 mediocre platforms is worse than 1 decent platform.
Over on mastodon, for example, there’s official mastodon … and then there’s a few forks of mastodon (eg glitch and hometown), pleroma and its fork akkoma, misskey and its forks sharkey, and firefish, and then forks of firefish, iceshrimp and catodon.
And yet there’s a good amount of conversation about how more platform diversity is needed to compete with mastodon. None of the above mentioned alternatives are especially stable, or perceived to be stable (firefish for instance was very popular in 2023 but has literally burnt to the ground with the main dev just abandoning it and the main instance), and so instance admins tend to be wary about running them and so they don’t get popular.
If half of the devs working on alternatives just banded together to make a compelling and stable and committed “alternative”, the fediverse would be in a better position.