Because up until the latest M3, all prior ones had a pin that wasn't captured.
The M4 is suppose to be easy to field service. Kinda like M16 push pins are captured. If you lose parts, the gun can shoot. It is easier to lose the M3 (or M1, M2, etc) clip. Your regular M1014 isn't going to be taken apart all the time to put in parts; just disassemble and oil.
Even if you happen to break or lose the M4 clip, the gun can still be assemble and shot because of the captured pin.