By looking through the install manual and parts it has to be related to the replacement steering knuckle. My first guess would be the new location of the tierod end in comparison to the wheel. The new knuckle raises the tierod location up quite a bit, and when doing so it also puts in further away from the wheel centerline which would require larger wheels to clear.
In regards to some other posts, if a stock vehicle can run 17" wheels than it obviously has nothing to do with the brake caliper clearance now needing 20" wheels.....it's in the exact same spot relative to the tire and wheel as before!
Also, it's not like a group of Rough Country guys were sitting around a table and just randomly said "hey, let's say you have to run 20" wheels just for the hell of it". Say what you want about parts manufacturers and even Rough Country, but why would they say this if it wasn't true and they found some fitment issues with smaller wheels? Granted, there could be an easy work-around to run 17s and maybe even some models could fit them with no modifications. But they obviously ran into some issues that required 20" wheels at some point in time.