One small problem is that the SMG is really handy for close-combat and portability in a way that (most) rifles aren't.
But we don't have any sort of close-in mechanic for awkward shooting, thus, a hold-out pistol's no better than a sniper rifle, and a sniper rifle is no worse than a hold out pistol, when you're wrestling with somebody. A GM might rule otherwise based on, you know, reality, but, some people get a tad angry that there's no rule that says you can't.
So, that takes away a reason for the durn things, leaving them less-concealable than a pistol and less powerful than an AssRifle, so, nobody really touches them.
(Sniper rifles, as they stand, are a real bugbear for me, due to being the best weapon around.)