A big issue with federated or, going even further, P2P alternatives to centralized platforms seems to be monetization. It's not so long ago that everyone seemed to be wondering if FB could turn a profit on their platform. What can an open, decentralized system do for revenue? Donations? Commercial tech support for node operators? It must pale in comparison with FB's advertising revenue.
This means a decentralized system can't beat a centralized one at marketing. Even with large resources, beating an established competitor is hard, but when the competitor has, and will always have, a huge edge in funds they can throw at user attraction, the problem seems insurmountable.