Of all the cartridges that have been developed over the past 20 years, many have fallen, and a few have gained enough market traction to become mainstay.
Every cartridge except for one was supported out-of-the-gate by big US manufacturers working in conjunction with a rifle manufacturer usually. To me, the story is more about the actual rifle market than the cartridge market because rifle manufacturers had more control over what would be adopted, since not everyone could build or assemble a rifle in their home.
Prior to that, the standard roll-out of a cartridge in the mid-late 1900s was for a big rifle company (that also made ammo) to launch a rifle and ammunition product combo together in many cases of successful cartridges. If the action already existed, they would introduce the new cartridge in legacy action/styles, as well as a new model for that action. The Model 70 Winchester and later Remington Model 700 were the most popular turn-bolt rifle actions of the latter 20th Century. There would be short and long action versions of each to accommodate the particular Cartridge Overall Length of certain classes of cartridges.
There was the battle of the belted magnums between Remington and Winchester, with the 7mm Rem Mag and .300 Win Mag emerging in each of their own spaces as the 2 dominant magnums. In the .473" case head long-action cartridges, the .30-06 and spin-offs became popular.
There were some great cartridges like the .264 Winchester Magnum and .284 Winchester that never gained wide acceptance though, while bringing real performance gains to the consumer that should have been better marketed and accepted.
The .30-06 Springfield before them became mainstay because it was the standard US military service rifle cartridge.
Same with the .308 Winchester and .223 Remington later on. You start to see a back-and-forth game that developed between Winchester and Remington in the 20th Century.
Fast-forward to the 1990s and 2000s and the proliferation of the AR15 in the wake of political pressure driving people that would never have purchased one, to everybody and their brother rushing to buy them because, "You never know when they will ban them outright, so get them while the gettin' is good."
An enterprising wildcatter named JD Jones of SSK Industries developed the .300 Whisper on his own, chambering TCs and AR15s in it primarily for suppressed use, with a heavy emphasis on it being necessary to fit and feed from the AR15's magazine and action with nothing but a barrel change.
After the .50 Beowulf in 2001, the 6.5 Grendel and 6.8 Remington SPC were developed in the early 2000s, one from a small company (Alexander Arms) working in conjunction with Lapua of Finland, while the other was done with the full support and backing of Remington in hopes that they would get a military contract for the new cartridge, since it had been advertised to them as something Special Forces was going to adopt.
The big difference now from a market standpoint was that individual customers could drop a completely new upper receiver chambered in a new cartridge onto their existing AR15 lower receiver group, slap in a magazine, and enjoy more energy on-target than the .223 Remington or 5.56 NATO AR15s, which were largely prohibited by most States for being used for hunting deer and medium or large game.
Ruger introduced the .204 Ruger with rifles to fire it in 2004.
Hornady introduced the short action 6.5 Creedmoor in 2007.
Remington later put in a lot of work on the .30 RAR (2008) to increase the power one could deliver from an enlarged upper receiver AR15 that still used AR15 lowers and most other parts for that reason.
The market started to shift from one where rifle manufacturers get to determine the actions and cartridges for each new season every few years, to one where consumers could switch to their pet cartridge faster than you would normally see any market be able to respond in the later 1900s. Since the AR15 is so modular and doesn't require traditional gunsmithing skills, with a reasonable degree of mechanical aptitude, the home DIY builder can assemble an upper themselves in whatever cartridge they can get a barrel, bolt, and magazines for.
So even though Alexander Arms made complete rifles and uppers in 6.5 Grendel, they weren't and still aren't a large company that is going to compete with rifle sales like Remington or Winchester. So what is driving the success of the 6.5 Grendel?
* AR15 compatibility
* Performance
* Ammunition availability, especially affordable ammo and a lot of hunting load options
* Steel case ammo
If you'll notice, none of the popular US rifle cartridges outside of .223 Rem and .308 have enjoyed steel case availability other than 6.5 Grendel, until recently (Dec 2019) with the new Wolf .300 Blackout Polyformance.
Anyway, the big rifle manufacturers who also make ammo aren't going to be champions of someone else's baby, and their norm is to fail at marketing most of the cartridges they introduce to the market, mishandle the execution of design and production of cartridges they do launch in many cases, and miss major opportunities to promote very successful cartridges that people have been begging for over decades. Look at the .260 Remington, for example. The .260 Remington and a wide line of Rem 700s and R-10s should be the flagships of the Remington brand. They don't even make many relevant loads for it. Hornady has better .260 Rem ammo, and the 6.5 Creedmoor is their golden child.
So if a company doesn't even do a good job supporting their own children, we shouldn't wonder why they don't advertise much for our pet cartridges, and just let the market do its thing. 6.5 Grendel sells itself by being AR15-friendly, with actual performance gains that no other double-column magazine AR15 cartridge has been able to match. To beat 6.5 Grendel, you have to go to one of the WSSMs in a single stack, with a different upper receiver, and not enough room in the COL limits of the magazines and mag well to have a nice 6.5 WSSM AR15.