A really popular mobile Jabber client will suddenly have the extensions it supports become popular with everyone else. Conversations looks nice.
As for why some set of extensions hasn't been brought up to modernity, that answer's simple: nobody cares about xmpp enough. Maybe just some users, but fuck those guys, they don't build anything.
Man, xmpp has been out for twenty years now.
Until you have two popular apps that implement two different extensions that accomplish the same thing. Now everyone is stuck implementing two, three, four extensions to display the same content. God forbid there's a new version of an extension that's backwards incompatible.