To be honest I don't know their reasons but from my personal perspective comparing the likes of the low profile PC to my slickster by Ferro the Ferro one was bar far the better PC. Better material use, Comfort and load baring support on the shoulders/body - felt like the WAS one would chaff with excessive use (sounds like a bra now).
Personally after using things like the cheaper chestrigs - Emerson/TMC etc and having had the Spiritus Systems micro rig I will always go back to the mayflower rig I have now (regret selling my first one) its comfortable, holds enough of what i actually need and can be mounted on a PC if needed.