Folks could say this kind of thing about most cartridges - 308 seems a little pointless when 30-06 and 300wm are out there - and 300 wsm, 300 H&H, 300 Dakota, 30-378, 300wby, 300PRC, RCM, RSAUM, Blackout, .30-03, 303 British, 308 Norma, 300 Norma Mag, 30-40 Krag, 7.62x54R, 30TC, 30 Nosler, 300 RUM… 7mm-08 seems a little pointless when 280 Rem and 7RM are out there, and 7wsm, RSAUM, PRC, 28 Nosler, 7 Practical, 284 win, 7x57 Mauser, 7 Dakota, STW… 223 Rem seems a little pointless when there’s 222 Rem, 222 mag, 22-250, 221 Fireball, 22 Hornet, 220 Swift, 22 PPC, 223 wsm, 224 valkyrie, 220 Russian…
It’s also fair to point out the fact the 6 BR isn’t a factory supported commercial cartridge. I’m not aware of any common brand factory rifle besides the Savage PTA 12 BR’s which are offered in 6 BR, and you’re not buying factory ammo from a shelf at common gun shops, let alone at Walmarts. Whereas 6 ARC has already been seen on shelves more frequently in its short life. The 6 GT also doesn’t appear to have as many factory rifle options available yet even as the 6 ARC - certainly easier to adopt for most brands than the ARC with its less-common boltface, but we haven’t yet seen big brands adding the GT. Soon, we can be sure, but not yet, while there ARE factory options on the shelf for the ARC. Ammo has been more broadly available for the ARC as well, although I might believe brass availability has been at relative parity for the two thus far.
Options, we want, so options, we have.