Accuracy and recoil bro, LMG's have the worst of it. Assault rifles have an advantage in ALL ranges, short/medium/long. LMG's also have terrible hipfire accuracy. The only thing LMG's have to compensate is a fatter magazine...but that's just not good enough.
Don't get me wrong, a good player will be able to wreck newbies with MK60/OGR easily, the huge magazine makes it quite a killing machine in close-midrange and you can burst fire for longrange. But it's still surpassed by all assault rifles in everything but magazine size.