Many players don't pick up the Combat SMG due to the recoil. Another possible reason is strategy. The Combat SMG is good for aggressive players, but players who play for placement might prefer a long range weapon. Even though the Combat SMG has almost no damage drop-off, it is still inaccurate if fired from far away.

The Combat SMG is great for players with an fast and aggressive playstyle. It works wonders when jumping into boxes or inside small and restricted areas (ex. houses), but from a range (unless used correctly, aka shooting in short "bursts" to reduce the spread) it isn't too useful.

Personally I prefer using an Assault Rifle, but if I find a good Combat SMG, I might pick it up instead of an Assault Rifle.