XMPP is not technically well-suited to mobile devices. It was designed with a base-level assumption that any client will maintain a constant TCP connection in order to receive messages. This doesn't fit well with mobile devices, which spend most of their time 'off' and try to have as little software running concurrently as possible in order to save battery life. Some mobile XMPP clients have come up with various clever workarounds, but all of them are hacks. For example, by keeping the 'real' XMPP connection open indefinitely on a remote, third-party server, and then communicating with the mobile app using a custom proprietary protocol that doesn't have this assumption.

A new protocol is needed. In fact, just the other day there was a story on HN where the original creator of XMPP said the same thing: https://news.ycombinator.com/item?id=6849755 The merits of his proposed solution are another matter, though.