And either gun doesn't use a magazine compromised with the x39 case taper - which forces a curved stack magazine to get doglegged into a straight mag well.
Goes to - just because you can shoot x39 in an AR doesn't mean you should.
.30 Rem AR also doesn't use AR compatible bolts or a compatible upper. Moot point about trading off the AR platform when it's as different as the AR10. It won't pin over to the other.
Search for others chambering in it and you find it's a stand alone. Remington only for parts and ammo.
The market for guns is like cigarettes - any little variation can sell, but when it comes right down to it, a smoker will usually take whatever they can get in a shortage. When severe economic or political circumstances pressure the industry, many of the slow selling and small profit calibers disappear completely, some never to return. Popularity counts, not necessarily technical differences just for the sake of becoming a forced single source of supply.
Trading off the popularity of .30 caliber just to sell a few guns is a niche market - selling to the old tech school thinker, not the future users that will be common in twenty years. They may very well be loading caseless, and brass case fed weapons as antique as blackpowder is today - well, if you still pour loose powder. New BP guns are shooting sabots over solid propellant, aren't they?
We're just one step away, either way.