Some folks are making this Army versus USMC utlization too complicated...it's not.
Army is basically mechanized infantry, and ride in vehicles on patrol. They need carbines because of limited room.
Marines are basically foot soldiers on patrol. And they are taught to use the rifle butt in M2M combat. There may also be an element of getting a "good cheek weld" for marksmanship, but I doubt that is valid, based on range reports comparing the 2 weapons.
I fire an Armalite AR15A4 National Match rifle in competition, which is nothing like combat, but can say that the nearly 9 pounds seems unwieldy compared to the 5 pounds of my old WWII M1 Carbine...if I were young enough to patrol in combat areas I'd want to be carrying the M4 Carbine!