My guess would be because the choice was made to integrate the front sight, gas block and bayonet lug into one unit. The bayonet lug needed to be set back far enough for... well... a bayonet to be affixed, and the gas system would be less reliable if the gas block was located at the muzzle (shorty Commando style carbines and Dissipators with rifle length gas systems are both known to be somewhat more prone to finicky operation because of this).