Things that spring to mind:

- Gas in an open space is the main one. It would need to be very potent stuff to be effective and something that potent in tiny doses is almost certain to be highly toxic

- Getting the gas widely disbursed in a built-up area may only be effective by destroying the buildings (so we're likely back to square one)

- Logistically a challenge as your own troops/transport would need to be quite close to take advantage (whereas dead people tend to stay put)

- Even if you could solve 1-3 it probably takes way too long to zonk someone out and it can be easily stopped by putting on gas masks or retreating.