The military could invest in compact, easily constructed tents or it could rely on the possibility that mud is available in the area & that they have the time, the personnel & the heavy equipment free to build them. Huts that you can't deconstruct and take with you.
Also we're not just talking about military purposes. You could put up an industrial grade tent in an hour for disaster relief. How long would a mud hut take to build in a disaster area? How about at a festival or fairgrounds?
It looks like in some areas the army is re-purposing tents for semi-permanent housing. Tents are already on hand. Their specs are known. I am not aware of an army approved mud hut. Perhaps the army should look into it, but there are many varying factors that can go wrong in constructing a mud hut vs a standard issue tent.
Also I don't think a quickly build mud hut is going to fair a whole lot better than a tent to a mortar round. Additionally the tents are being sprayed with insulating foam on the outside which makes them almost airtight. Though I'd question if there is the ability to re-purpose the tent after it's been sprayed.
Rammed Earth construction (http://en.wikipedia.org/wiki/Rammed_earth) is an ancient practice made easy with modern equipment. The tents the American forces use are better suited to a European climate, not the intense heat of the desert. They're also going to resist mortars a lot better than some flimsy canvas tent.