Because tons of moving parts easily get dirty and jammed in the field, hard to repair, and as well probably because firing from the hip with an LMG is horribly inaccurate, not to mention that soldiers dont just stand all the time, they need to get prone or crouch in an instant and the contraption looks like it inhibits movement
well going off of the aliens movies and games, the smart gun locks onto its target via helmet visor, and it fires caseless ammunition, so less recoil. I can see what you mean about dirt, but as long as its cleaned it will work fine, most gun malfunctions are due to negligence. But I don't mean for this to be the weapon to replace all LMG's just simply a weapon for special forces for CQB. As it has no problem maneuvering around tight corridors and firing around them either, as shown in the movies and games and the video above.
