Vendors in that market heavily use extension mechanisms. For instance commanders of infantry units might fill out forms every day about what kind of contacts they had with the enemy and get them sent to a central location.
How does XMPP help with this over a HTTP form?
But: - HTTP forms might be less flexible when it is about structured data. - HTML needs a complex renderer software
Remember that this stuff might need to work on low resource devices and over low bandwidth connections.
HTML forms are pretty flexible and could be designed in a low CPU environment.
XMPP Forms are deeply asynchronous. I'd assume that the "blank form" already exists on the soldier's laptop and the message gets stored-and-forwarded until it gets to the destination.
HTML is an async protocol. We pretend it is sync through a mountain of cookies. But innately, HTTP and HTML are all async. Without PHPSESSID, there really isn't an official way to correlate GETs with POSTs.
XMPP is synchronous. It's a chat interface. I'd assume it's be harder to do async comms with it. IIRC, it's a stream of XML that coincides with the servers state.