XMPP _was_ bad on mobiles. The biggest issues were a) it was overly chatty (i.e. constantly sending/receiving information, draining your battery) and arguably

XMPP _was_ bad on mobiles. The biggest issues were a) it was overly chatty (i.e. constantly sending/receiving information, draining your battery) and arguably worse b) it wasn't designed for changing addresses (wifi->data and back) or constant loss of connectivity. So if you used XMPP some years ago you'd kill your mobile, fast, and would constantly lose messages. Other features of modern IM solutions (say, an archive of your messages?) weren't present or usable either. Today? Most of these issues are resolved. Prosody is a great server to get started (again) and comes with solutions for most (all? MAM aka 'Message Archive' is still somewhat scary imo) problems I mentioned above. Conversation is a very nice and modern XMPP client. TL/DR: XMPP was 'bad' for mobile deployment for a long time and probably lost too much traction because of that. XMPP isn't bad in that respect anymore.