what does barrel length have to do with accuracy?
hint: nothing (if anything increased stiffness helps it).
Reduced velocity is what you get from shorter barrels. If you're shooting subsonic ammo anyway, it literally doesn't matter.
For 300blk subsonic, a bolt gun is quieter than a semi auto or even an AR with the option to turn "off" the gas. That's what a lot of people like about the bolt action 300blk. Pistol is just popular because if you're not gaining anything from longer barrels (ie. velocity) then why pay the weight and length penalty? And instead of SBRing a rifle, you can build a pistol, so - that's the reason you see pistols.
For 308, etc - the reduced velocity just cuts down the useful range. If you're hunting and your shots will be within 100-200 yards on deer, then you don't need a 16-20" 308. Smaller is better in blinds and stands, and lighter is better carrying around. Shorter also means you can suppress it without having a musket-length rifle. Again, if a 12.5" 308 will give you the ballistics you need for a 200 yard kill, and that's all you need, why pay the weight and length penalty for nothing gained? And an AR, even a short one, in 308 is going to be way heavier than a bolt action of the same barrel length - and longer, too, typically. So if you don't need semi auto capability, why pay the weight and length penalty?
Short bolt-action pistols aren't going to replace standard rifles. They aren't better at everything - they are a specific application for a specific purpose. If you have such a purpose, then maybe that's just what you were looking for. If you don't then you'll go with something else.
But accuracy isn't even part of the equation. Accuracy between models, designs, calibers, individual samples, etc is to be considered just like in standard-length rifles, but just because the barrel is "short" doesn't have any intrinsic detrimental effect on accuracy.