They don't work for the specific task of social networks, not saying they are useless, I think Matrix is a great use case but that is for instant messaging.

Social networks need large well connected "networks" to scale. They need permanence, persistence and discoverability. Having 1000s of different home instances is very cool for decentralization but it hurts discoverability and thus normies don't use it.

If the ecosystem collapses on a few handful big instances, it just stops being decentralized so might as well give up on the federation concept.

As the network scales, so does the amount of content that needs to be hosted there. Who will be willing to host all of that in the future for free?

The fact of the matter is none of these fediverse type social networks did really take off. Maybe I will be wrong in the future but at least now that appears to be the case.