It's definitely not the lack of features, since XMPP is extendable. I've heard that the protocol is quite chatty and would cause power drain on phones. That could probably be solved with a binary serialization alternative to XML, though.
As for a binary serialization, XMPP traffic does compress very well already (TLS/zlib/lzw), but we have started the process on standardizing the use of EXI (https://en.wikipedia.org/wiki/Efficient_XML_Interchange).
However, the XMPP community has been active in quantifying such issues and providing solutions. The beginning of a document with background information is available as http://xmpp.org/extensions/xep-0286.html. There are also various opinions on the topic to be found online, like http://www.deepdarc.com/2008/02/14/mobile-xmpp/, that, as you can see, dates back half a decade already.
Facebook also have a private extension, as I understand things.
I should actually submit that extension - I wrote it up as a XEP ages back, but various things have held it up.