Bottom line is, the map size is so small that if you use a AR or SR you'll be generally at a disadvantage.
I am an AR user and I have no disadvantages in my game play, shooting, maneuverability or issue with smaller maps. SMG are simply easier to use, unfortunately thats what the devs choose to do.
Its general usage thought is limited, the one advantage they had over SMGs was effectiveness at medium to long range, they simply don't have this advantage in this game. Sure you can still do well but stat wise and general feel and from what many witness in the community, AR are generally less used because SMGs are easier to handle and the map size allows them to be majorly effective.
Take this games Context and then put it inside MW2 maps, we'd see many people struggle to use these SMGs in a map like Quarry or Derail.
See what I am overall getting at?