As it stands, the AT4X pack includes a set of 33-inch Mud Terrain tires (LT285/70R17C) as standard. So it could very well be that GMC saw little demand for Mud Terrains on the regular AT4 ...
BFGoodrich All-Terrain tires have seen just about every kind of road, mud, dirt, and sand you could ... with popular sizes in the 33-35-inch range fitting 16-, 17-, 18-, and 20-inch wheels.