Comment by brimble

Chemical weapons are expensive. Consider the logistics and training required to effectively deploy them, plus any specialized equipment. Meanwhile, they're only useful as long as your opponent doesn't know you're planning to use chemical weapons, since countermeasures are relatively cheap and every major military knew what to do about them by the time WWII broke out. As soon as your enemy knows to beware chemical attacks, all you're doing is annoying them while making it hard for your own troops to advance (they have to put on chemical suits/masks themselves, or else wait for the gas to disperse). Very hard to use effectively in maneuver warfare. They didn't even prove very effective in WWI, which was much closer to an ideal environment for their use.