One of the main reasons IMHO is that it's a ridiculously over complicated horrible badly designed hideous protocol, which was designed by committee rather than organically grown.
Writing a bot for XMPP isn't a simple matter of sending a few plain text commands. It's an absolute nightmare. It's so verbose and overcomplicated it's just not funny any more.
(I'm currently working on XMPP support in Mibbit, and writing the protocol lib myself so the pain is still raw).