There are lots of reasons for a lack of these things on the market.
First, tho, I do not know that it WAS a good gun. As stated, it had stiff competition off the blocks and it had better have been a very, very good gun to have beat several others. It didn't and maybe function was a part of the equation...?
Also, redesigning an open bolt subgun to closed bolt firing to meet BATFE requirements can be much more difficult from a production standpoint than might be thought at first blush.
In essence, the gun {I've never seen the internals on one close enough to know for sure} might for all intents and purposes require such a radical redesign that it would require substantially new tooling, etc.
This thing is dead.
I owned a couple Stemple .45 ACP submachine guns {full auto} and had some fun with them for a while till they became boring, but as for semiauto pistol carbines, well...really, those who have owned semiauto 9mm's know what popguns they are. For the cost, I'd rather be shooting a .22 LR.
I think most who buy one {like I did} get bored with them and wonder what they can do with them that cannot be done 1000X's better with a carbine shooting a cartridge of even the 5.56 or 5.45 or 7.62x39 performance level. Or a .22, or a heavier "pistol" caliber like the .44 Mag.
Musing some more...
In my opinion, pistol caliber carbines only become useful, at least for the practical puposes we put them to, when they get into the .357 Magnum {minimum} ballistics range, on up. My little folding stocked .44 Magnum carbine works for me, but I would not want a 9mm or .45 ACP carbine at all. No matter how cute it looked.
Maybe I'd extend my horizons a bit to include a sub-caliber PDW-type like the P90 5.7 or the HK 4.85 as those rounds are flat shooting enough for some varmint work, but again, the wimpy rounds like 9x19 and .45 ACP hold no interest for me in a carbine, and evidently they don't for nearly everybody else, too.
Shoot a 9x19 or .45 ACP carbine at even 100 meters and you quickly learn just why even the sub gun has gone the way of the dodo.