This is pertinent to this thread since I see a parallel between the arguments about 6.5 Grendel/ 6ARC and the older one about the 6.8 SPC/Grendel.
To answer the question: the increasing popularity in the 20-teens of shooting steel at longer ranges and sudden interest in 6.5mm cartridges in the US has increased the popularity of the 6.5 Grendel. Although it is hard to believe now, even 15 years ago there were articles with titles like "the unpopular 6.5mm". For whatever reasons, Americans traditionally did not like 6.5mm, but the 6.5 Creedmoor changed their minds over a few years.
Early on in both the SPC and Grendel's lives (2005-2010), there were more bullets and factory loads for the 6.8 SPC, largely because it was briefly seen as a potential military round and possibly because .277" was seen as "an American Caliber". The bullet and ammo situation has reversed in recent years. The 6.8 SPC is still a good hunting round, but the average shooter now spends relatively more time shooting at steel targets beyond traditional hunting ranges, something at which the Grendel excels.*
The 6mm ARC has been an extension of that trend, where ringing steel at longer ranges has a higher priority than killing deer or hogs. In some respects, it re-created the old, not-quite-true SPC/Grendel argument: "the former is better for hunting, the latter better for targets". As mentioned above though, the introduction of the 100 gran ELD-VT has improved the long-range picture for the Grendel.
*Yes, there are some bullets that can allow the SPC to hit targets at longer ranges, but then again, there have always been some good hunting bullets for the Grendel. Avid proponents for each cartridge tended to ignore inconvenient facts like that, though.
