I realize it's a rhetorical question - but I think it goes like this: matrix was created to fix issues with jabber/xmpp - both fixed issues with IRC. All three are open protocols.

Zulip was created with a focus on keeping history as usefully as possible - mattermost as another open chat platform.

Except for timing - mattermost and zulip might both have been built on matrix (the servers and clients weren't there yet when theses projects were started).

That might be the case for Revolt too.

Personally I'd love to see more innovation on top of matrix.