It's because platforms can deal with feature complexity and UX standardisation in a way that protocols can't.
Multi-protocol clients tend to end up a mess compared to the integrated experience of a platform which can provide a single source of truth for identity, authentication, and so on.
Netscape Communicator ticked many of the boxes of Facebook years earlier, but by kludging together NNTP, HTTP, SMTP, POP3, FTP etc., and that's before you consider the difficulty of moderating an open syndication like Usenet or IRC, or the pain in the ass that email spam had become by the early 00s.
Protocol/standards people like to think they care about UX, but for platform companies, user growth and retention literally pays their bills. It's just a different set of incentives.
And to be clear, I prefer the more open internet, but UX wise, it never stood a chance against normie-optimised, integrated platforms.