I've actually been pondering this lately after being reminded of .30 Super Carry and here's my conclusion...
I believe that .30 Super Carry failed to catch on solely due to marketing... Yes, that's really what it all comes down to, and it isn't an oversimplification. Allow me to explain...
It seems evident to me that .30 Super Carry was designed for the sole purpose of appealing to marketing trends, with its very specifications being built up around statistics. Unfortunately, marketing executives have a bad habit of dehumanizing consumers and thinking of them not unlike cattle, ergo they are incapable of seeing the rhyme or reason behind marketing trends and just look at it like this; "It has been observed that this is what the animals like to eat, so if we mix all of these ingredients together, the animals will like them even better and come to our barn for feed."
What they fail to recognize is that each of those little things that are popular in the market right now have a form of rationality behind them and not all of them go together nor are they shared by everyone.
So the end result is a cartridge which lacks any tangible benefit because it's built up upon a mishmash of marketing trends which do not necessarily collectively appeal to the target demographic, nor do they offer anything new or compelling.
Nearly every one of .30 Super Carry's benefits are offset by the fact that they are shared by other cartridges.
One of the .30 Super Carry's most readily apparent benefits is High Capacity, but 9mm Parabellum already has this covered, and while .30SC does offer higher capacity, how often do you see people complaining about 9mm Pistols offering too little magazine capacity? Pretty much never, because it's the gold standard, hence why other cartridges offering higher capacity like 5.7x28 FN never really caught on.
So without that, what does it bring to the table? Pretty much nothing, because it was designed around 9mm to begin with and thusly does nothing better whatsoever. Similar sized pistols and similar muzzle energy, but without the years of service by numerous Military Factions and Law Enforcement Agencies to back it up.
That's also I think what one of the greatest marketing missteps of the cartridge was as well. Obviously, many prospective new buyers of firearms look towards the Military and Law Enforcement for examples of what might be a good defensive cartridge because it's comforting to know that what they carry is used by soldiers and police officers. They don't want a civilian carry cartridge, they want a duty cartridge, ergo .30 Super Carry isn't going to appeal to them. Had it been marketed as a Duty cartridge under the name; ".30 Super Duty" or ".30 Parabellum" then aggressively marketed towards Law Enforcement with absurd discounts just to get somebody to adopt it so that they could in turn market it as; "Proudly serving the Podunk Police!" with an endorsement by Deputy Barney Fife, then they might at least be able to trick newcomers into buying it.
As for those who are already into firearms for self-defense, it cannot appeal to anyone in the short term because everyone is already settled on what they trust, so unless it offers something new and compelling, nobody is going to rush out to buy it.
.30 Super Carry needed a stronger selling point, something unique to set it apart from other cartridges, then it would have stood a chance of catching on, but it wasn't designed for any particular purpose, it was merely designed to appeal to marketing trends without any understanding whatsoever of the motivations of consumers which set those trends in the first place.