I believe its 2 fold:
1. Fox suspension holds more prestige, generally like for like a consumer will believe Fox product to be an 'upgrade' vs. RS. Fox's performance elite line especially, delivers all of the same function of the top end factory line only giving up a cosmetic outward appearance. With RS, oem's are having to move to a lower end RC charger damper or even into the age old motion control dampers to deliver on the value end.
2. Makes the consumer think the manufacturer is on your side. Sure they may have been able to save a few bucks speccing within the same family. By going outside the SRAM umbrella, we feel that they have gone the extra mile to deliver a more complete package when the functional gap is likely much smaller.