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's messages, as if they had been talking to themselves. (Yes, someone did eventually write an extension for that, which nobody ended up supporting.)
I can't even say that XMPP failed to adapt to mobile. It'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.
I can't even say that XMPP failed to adapt to mobile. It'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.