Because availability of a variety of .308 projectiles and an almost infinite supply of parent brass.
This. The way I see it, is why bother with trying to make the x39 work in an AR, when you can just use the 300 Blackout? I load my own ammo, and don't see any advantage to the 7.62x39. Brass is harder to find, bullets are an odd size, and it uses more powder.
