While a certain cartridge may not yet be on your local shelf, rest assured, that is just a data point of one, not a trend. And, vice versa. As the public learns of new rounds, they also learn of new firearms to shoot it in, as that is also required. Very few hear of a cartridge and then rebarrel an action to accommodate it. If there were 6.8SPC barrels available for the Win 94, they would sell.
What we have now is the AR15 replacing that venerable firearm, especially in light of it being used for over 45 years and over 20 million able bodied servicemen and women using it. It's a new generation - and we can't get M16's from the DCM. If you want to own the firearm you served with, you buy an AR, and in some states, you get alternate calibers because 5.56 isn't hunting legal. And while there are a lot of wildcats getting stuffed into AR's, the baseline of performance to compare is the 6.8SPC. It has something to do with being sponsored by Special Forces and designed by the Army Marksmanship Unit.
What has happened since is that it has been adopted by foreign governments for use by their armed forces. The cartridge is also mentioned as being on the short list for importation by foreign makers. It seems our political climate interrupts that consideration as they pursue more profit from the conventional calibers.
It, like 6.5 G, is also taking shelf space away from other rounds, which is the OP's point - some calibers ARE going away. In the market aftershock it was reported by a major ammo maker last spring they would suspend making certain cartridges. Those loads simply didn't sell well, the time lost in setting them up would make many more rounds that were already sold in other cartridges. It was more profitable to just leave the machines running.
There are different ways to look at it - from the mass merchandiser perspective, there are a certain number of feet of shelf space, it's limited, and when a cartridge falls below a certain profit point, another with better results will be inserted into the lineup. From the makers perspective, it's easier to get into cartridge manufacture now, smaller companies can concentrate on the lesser known ones as a niche product and keep their machines running. The expansion of shooting and demand have risen enough to make them money, where the sales would have been a loser to a major maker.
So, we see cartridges fall out of favor and come off the shelf to be replaced, while the internet keeps the smaller makers alive and the cartridges don't completely disappear into the pages of the reloading and wildcat manuals.
