It’s the pistol grip. Much easier to carry an AR-15 muzzle low. M1’s only had a stock grip and it would be akward to carry it muzzle down. I also believe it’s POLITICALLY CORRECT, in that it appears you are not ready to shoot the hell out of anybody if it’s pointed down. Just my 2 cents from an old vet.
Spoken like somebody who has carried a version of both.
Pistol Grip, Stock, Stock weigth, barrel length, etc, etc.
All anybody has to do is get a copy of each style and do a little point and aim in the back yard.
That pistol grip makes a big difference.