Possibly that's because in competitive play the Stalker perk is banned and the MTAR-X is the closest thing you can get to an AR with Stalker? I'd assume that's at least part of it...
I really don't think there's a N00b gun in this game TBH. The AK-12 is obviously popular but doesn't kill quickly enough in CQC to make it OP; same with the Remington. The MSBS was initially popular but (as someone else stated) is MIA after the nerf. FAD and ARX-160 have too much recoil to be widely adopted; SC-2010 has almost no recoil but is otherwise very bland. Honey Badger is good up close but sucks at range. Finally among ARs is the SA-805, which I personally love and play with almost all the time; but something about its wobbly recoil (and the fact that its iron sights are, like the MTAR-Xs, basically unusable) seems to turn a lot of people off.