I think a lot of folks assume the 6 ARC is highly popular because of the hype over covid era, but it really seems like more folks are buying 350L's so far than 6 ARC's, and I might acknowledge that the flood for 224 Valkyries was very violent when it hit, so at this point in time, there might still be more Valkyries running around than 6 ARC's yet - and were it not for the 22 ARC, that might not have ever changed.
But the mini-length cartridge market is what it is - we had a reality for a long time where we had 5.56/.223 was our only option, and then for several years, all we had was the SPC and the Grendel, with the Beowulf being even more obscure option than the Grendel from Wild Bill. Then we got the 458 soc and 450 Bushmaster, the 300 blk came around and that's all we did for a while, we rolled in a few easy ones like 7.62x39, 204 Ruger, 17 Rem, etc, then the "long range AR cartridges" in the 22 Nosler, 224 Valkyrie, and 6 ARC on the heels of the long range revolution, with the 350Legend, and now 400L. We picked up more easy ones like 22 ARC and 24 Nosler, 6x45, 25-45 Sharps, we have the WSSM's, 6 Hagar for a few hold outs, picked up a few oddities like the 300 HamR, and a hundred other wildcats from a dozen different houses all built to the 2.3" action length... And the last decade has seen a HUGE emergence of pistol cartridge lowers, rifles, and conversions to serve a market which simply didn't exist before....
We've entered into the age of parody where we can talk about mini-length cartridges in the same way we've debated "which is your favorite magnum cartridge?" or "what is your favorite short action cartridge?" and actually have enough players to almost be interesting. 25 years ago, if we talked about "what is your favorite AR?" the only aspect of discussion was barrel length... 20yrs ago, we were still cutting A2 FSB's and drilling and tapping to make AGB's, still cutting A2 handles off of uppers and screwing and pinning on scope blocks, and gluing carbon-fiber tube stocks onto barrel nuts to make free float handguards. Today, it's an entirely different spectrum of options...
We have ammo catalogs online with hundreds of options, we have reloading manuals with hundreds of cartridges - we've entered an era where mini-length cartridges have emerged as a sizable minority among them, and an era in which more and more shooters are finding interest in AR's which are NOT in 223/5.56, and don't feel the need to own a 16" 5.56 as their first AR, or even own one at all.