For the longest time I though it was because of the weight..that on a SxS most of the weight is in the rear so to get the same pressure per square inch, you needed more contact patch area...so a wider tire was needed. But on an ATV, the engine in mid-frame...so..I thought it was because you carry more weight on the back rack then the front. And a GNCC racer friend said he always puts the same size tire all the way around because..he doesn't carry anything extra so..that like of thought started to make sense. But there are 4WD trucks...and because we rotate those tires, they all have to be the same...but..just as an example, my F350 with 6.7 has... a lot of weight up front...and to make the tires not look low I add 8-10 more PSI to the fronts then the rears. And I certainty won't ever put wider tires on the front. But all 4wd trucks work just fine with the same size tires all the way around so..why not a SxS. Just put a few extra pounds in the rear and go.