No reason for this other then simply not having a request for it (until now) :).
After some quick Googling, it sort of appears everyone uses this protocol differently with different payloads (I could be really wrong here and would love if someone were to weigh in here). For example Google Talk servers require you to define additional XML which includes a token, where as other services just require you to send the sub/pub standard XML. These differences in payloads would make it difficult to build a simple URL with (while following the current convention).
So rather than support XMPP, maybe it would be better to create a notification per service you want to attach too? Did you have one in mind?
Currently, I use a simple Go client [1] and a self-hosted XMPP-server (ejabberd). In fact, I don't know what the Google servers require, but due to the federated nature of XMPP (similar to e-mail), I think it would be best to create a generic version rather than one for a specific service.
