I've wondered that. I assume it's at least perceived supply and demand, as there area options for such with Benellis. Now that the 1301 has been the new hotness for several years, maybe that will change. There is one possible technical issue, which is that the gas piston uses the OD of the mag tube as a guide rod as well as sealing surface, and the tube seems to have more than basic anodizing on it (maybe a Teflon sealer?). I don't know whether that would be a show stopper for an aftermarket tube, or just nobody's tried.