Chances are its a third party vendor. The chrome in the Denali boards faces outward. The trim on the power boards faces down. I'm sure with the front wheels shooting rocks and whatnot toward the bottom of that power board, stainless was the way to go. The chrome would just chip.
Mmm I dunno, the oval boards that I just took off and sold were on the truck for almost a year ... after I washed them and windexed them the chrome was spotless and exactly the same underneath as above. Chrome, if applied correctly, is very resilient to the elements ... in fact probably more so than stainless steel, or at least just as much.