XMPP lacks a fundamental feature: it does not echo your messages back to your other devices. This made it uniquely unsuitable for the mobile world: if you start a conversation with someone on your computer, and then move to your phone, all you see on the phone are the other person&#x27;s messages, as if they had been talking to themselves. (Yes, someone did eventually write an extension for that, which nobody ended up supporting.)<p>I can&#x27;t even say that XMPP failed to adapt to mobile. It&#x27;s more that XMPP was a terrible protocol with a gigantic flaw that should not have been there in the first place, and mobile just made the flaw obvious to everyone.