"Industry support" isn't what makes or breaks a cartridge. The industry has been supporting the .30 x 5.56 for over 35 years - only now has Remington bothered to adopt it.
Entirely because the 6.8SPC paved the way to become the #1 alternate in the AR in the USA. There's more guns, barrels, and parts out there for 6.8, and more users than any other cartridge in the AR.
How? Hunting. It's where you make or break in the American gun market. If you want to sell a cartridge and keep selling it to new shooters, it has to have something good about it, and the 6.8 has 40% more power in the same range as the AR. That makes it a deer rifle, legal in all states.
It's a standard taper, bullet to case ratio cartridge, that feeds correctly in the AR straight magazine well, which makes it MORE reliable than the x39. If anything, attempting to get the curved mag feed of the x39 through the AR mag well is why so many haven't gone to it - it's the #1 example of why some things shouldn't be done. Please look carefully, the AK doesn't have a mag well like that, for that specific reason.
The 6.8 also has the same power levels as the .30-30, something the x39 struggles to approach. The American hunter has taken deer with the .30-30 for over 100 years, it's always ranked in the top ten in ammo sales, and it works - the effective range out to 250 meters covers what you can usually shoot hunting deer.
Here's another facet of why 6.8 has grown so much: it was developed by Special Forces with the assistance of the Army Marksmanship Unit. When it's said "they" tested different calibers, that means they shot things with it to find out. These guys aren't a bunch of wildcatters, they were working on making a better killing bullet with enough accuracy to do the job. You only need 2MOA out to 500m to do that in combat. Any further, the Army has crew served weapons to do it. The individual shooter in a team or squad isn't assigned that job, and will get knocked upside his kevlar for wasting the ammo.
No, the 6.8SPC wasn't designed to out Bubba the x39, that was already being done with 5.56. Since the Army didn't buy into it, fine, they have much bigger issues. But we came out ahead with a serious cartridge for making the AR a great hunting rifle, which is exactly why I chose it when I built mine. It's not going away soon, if ever, and continues to sell more every year. Ammo is even showing up on the shelf at Cabela's and Academy. It takes a lot of sales to get corporate buyers to switch to something new, and it has to do with making more money. They don't ever do it just to be nice.
Look into it some more, basically none of the preconceptions listed in the first post have any validity. It's not unusual, the average shooter gets fed a lot of baloney from misinformed friends and gun shop employees with sales agendas.
Russia ditched the 7.62x39 awhile back for a smaller cartridge much like the 5.56 of our AR.