Because the moment the wombo combo is gone is the moment that the Nightstalker grenades and melee charge become very subpar. The grenades by themselves are all pretty bad unless the enemy just stands there in it. And smoke doesn't really do much damage itself. The extra effects are what makes it useful. Expending both together makes it lethal. Which is the trade off. You have to use both of your charges for 1 kill. You also have to spec the Envenomed and Lockdown perks for it to be effective. And it does require accuracy if you want to do more than just wound your target, aka you need to land it at their feet so that they can't escape.