Because the mags would be illegal. Under the '94 bill, all detachable magazine shotguns are semi-automatic assault shotguns.[rolleyes] That is why you can't get the pistol grip version in the U.S. unless you are a leo. Shotguns mags over 5 rounds are illegal. Basically, that excellent shotgun is facing the same problems as the spas-15(another detachable magazine fed combat shotgun) faced. Probably doing great in other countries. I believe the largest mags made for any 12 gauge detachable magazine shotgun are 8 rounders. Both franci, and saiga have had problems making larger magazines reliable. I know saiga tried making larger magazines with more expensive parts, but the expensive 10 rounders were seen as not marketable due to pricing compared to 8 rounders. And I think the 20's were rejected due to size.
Too bad we can't have the real good ones. Short barrels, folding stocks, pistol grips, and select-fire. They make them in Russia, but would break way too many laws to import here.