I know it is unlikely that someone has a definitive answer, but I am curious as to what reasons users may speculate. I think these guns are quite nice. I have handled them in a couple calibers, in both gen1 and gen2. The prospect of owning one in 7.62x39 is appealing. Why on earth would they opt not to make the gas system adjustable? Seems like a no-brainer considering the proliferation of silencers in all the markets (LE, Mil, and commercial). Not to mention the relative ease of integrating an adjusting mechanism in a piston design. Most other modern piston guns have adjustability, and that's the competition for the ACE. As a refined AK, the closest comparison may be the SiG 55X, which had adjustable gas since it's inception, before the proliferation of silencers. Just seems like a bonehead move, on IWI's part.

I am aware of aftermarket solutions from KNS. I want to know why a reputable, competent arms design firm opted not to build it into the gun from the start.