Logistics. FAS still sells direct while KPA has relatively major distributors... which means in an "oh shit" scenario a tour manager can start calling the closest Guitar Center, Sam Ash, ect until they find a replacement.
Tonally it's probably close enough to get the job done while being acceptable in their IEM mix, and don't forget these guys have some of the best live sound engineers who can make a toilet sound great in an arena... and those guys have about a hundred other things going on simultaneously, so the easier they can make their life the better since 99%+ of the typical audience thinks sound comes from empty cabs on stage and they're happy with "loud."
The closer the band gets to relying on pushed clean tones, the more those subtleties we love start to actually matter in a live, large room context. I see a lot of metal bands rocking KPAs and a few with their personal FAS units (i.e., the gifts of Christmas crowd), but there's a reason you saw a little Princeton combo at the Super Bowl and why you don't see many KPAs from Mayer-ish players.