The lower/components, buffers, bolts, and magazines are more expensive than the 5.56 varieties.

The same price point would not happen.

A USGI rifle uses a 20" barrel.

5.56/223 often penetrates LESS in building materials than pistol calibers.

Most people buy 9mm AR carbines due to ammo cost, familiarity, ability to shoot at indoor pistol ranges, reduces noise/blast, and quite simply, because they are fun.