Because the smoke acts as a suppression grenade, which isn't a viable option alone if you plan to get kills with grenades. They do 15 ticks of damage every second for four seconds. Which is nowhere close to the amount needed to kill or be as effective as other grenades.
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I get throwing knives because it's a knife, but a smoke bomb shouldn't be the melee.
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A little difference in the game is always nice, doesn't always have to be the same y'know.