Jabber/XMPP has a number of problems that make it unsuitable for modern chat environments where clients (read: smartphones) have poor connections and frequently want to sleep to conserve power.

For example, XMPP wants an always-on TCP connection between the client and the server for the client to appear "online". There's an extension which allows for sleeping, but having async mobile connections in an extension means that you need the right pair of server and client to enable this feature. Plus, there's no handling of syncing messages between multiple clients. If you've got a smartphone and a desktop client, to compete with Facebook Messenger you need detailed conversation transcripts on both clients -- and an intelligent way to ping your clients and tell which should receive sound notifications.

see more at https://op-co.de/blog/posts/mobile_xmpp_in_2014/