
I'm seeing Rab next week, so I'll ask them, if I remember. I suspect though that the answer - in the real world - is that pile/Pertex, despite working extremely well, is just a unfashionable. I don't think they're that expensive to produce judging by the price of the Montane Extreme.
Patagonia produced several rather better looking shells using the same principle, but with better aesthetics and more sophisticated fabrics, notably the Stretch Speed Ascent which is one of my fave things of all time.
I'm convinced that a more designed, aesthetically pleasing pile-Pertex jacket would perform really well, but right now no-one seems inclined to make one.