The theory behind 17s is sidewall stability. It really came from the 37in Krawler and it's development (from what I understand). BFG was looking to optimize sidewall flex as well as side load stability in off camber/turning situations. The 17 was the best compromise. Now, when you step up to a 39 or 40 inch tire, you really need a rim that is something like 18 or larger to retain the same stability and performance. Aftermarket companies obviously don't supply odd ball sizes like that, so companies are limited to 17s or 20s. Notice that the new 42in Krawler ONLY comes in a 20in rim? Well, it's not because BFG wanted their sponsored drivers looking like ballers, it's because it needed it in order to retain performance.
