Comment by zamalek

I've heard from people who have tried to implement it: SIMPLE (3428) is anything but simple. It is a very good example of "enterprise design." In addition, as soon as you start extending the protocol there is a very good chance that your implementation will no longer be interoperable. Take Lync (Microsoft's implementation of SIMPLE) as an example: due to the presence of custom extensions, clients have to implement specific support for it - the protocol doesn't gracefully degrade like XMPP does.

Basically, XMPP came first (first foot in the door) and is a significantly superior protocol due to its simplicity.