because, with offroading you want more sidewall. more sidewall=more sidewall flex and thus the tires can wrap around things better and get the best grip. they are the smallest size the mass vehicles can use. bigger trucks can only use 16s or bigger so you will see more 17s and up on fullsize trucks.
Plus, smaller rims will make your tires look bigger too. And you've got less chance of bashing your rim on a rock or something with more sidewall. Also, the cost for a 33" tire that fits a 15" rim will be less than a 33" tire that fits a 16 or 17 inch rim.
I had to pay more for my 16 inch rim AT tires than i would with the same size and tire only with a 15 inch rim. But 18 inch rims on mudders are sweet looking. I've noticed Nitto & Toyo are coming out with larger rimmed AT & MT tires. As of now you can run a 22 inch rim on some MT's or AT's your choice.