Because open protocols were abandoned, because they generally lacked security, discoverability, and perhaps most significantly of all, moderation.

Even FINGER had .plan files, where you could get status updates from people.

But how are you going to query whose plans you want to read, and who is going to moderate without centralisation?

It's the moderation in particular that dooms most federalised efforts. The web itself is only really moderated through the relative lack of discoverability, with Google and hosting providers having become a form of gatekeeper to remove the very worst of the web.

Twitter in the early days took off because it did solve a technological problem of the time. You only had your SMS enabled phone, no WAP or smartphone, and wanted to provide updates. They gave you a number you could text, which would post updates. Hence the character limit ( less than the SMS limit, to leave space for your credentials ).

Plenty of alternatives exist for getting information out, people just don't know where to look for it.

That technical problem disappeared, but it had by then grown to provide a service to rapidly get updates from people, one that had a sense of authority and the moderating effect and discoverability of centralisation is impossible to ignore.