I would hazard a guess and say cartridge availability. Part of the appeal of the carbine/scout is general availability of ammo. 308 is one probably one of the more common rounds, especially in.30 caliber. Some of those rounds you suggest are nice, but really are not sitting on shelves in most gun stores. 308, 270, 30-06, 5.56...pretty much available at Wal-Mart.
Brent.
Can't argue with that! I have been reloading close to 50 years now so just do not worry about that.