The reason is not complex, it’s just rarely discussed. The first time Chlorine was used it killed a lot of soldiers, because nobody had a clue what was going on

The reason is not complex, it’s just rarely discussed. The first time Chlorine was used it killed a lot of soldiers, because nobody had a clue what was going on and no one was equipped or trained to deal with it. By the end of the same war, chemical weapons killed very very few soldiers, because they had training, and masks. What a chemical weapon does really well is kill civilians. In particular it kills the young and old who can’t get away, who are weakest. Chemical weapons can render an area uninhabitable, one thing they’re terrible at is killing soldiers. They have MOPP suits, and training, and vehicles with filters and overpressure seals, and autoinjectors with reversal agents. So why are chemical weapons banned? Well the talking heads like to say “they don’t work, they’re just as likely to blow back on your own troops, they’re cruel.” Really though, they do work on non-combatants. Intentionally targeting non-combatants is what’s actually illegal and discouraged. Dropping bombs to destroy a city or AA batttery while knowing that you’ll massacre civilians is seen as tacitly kosher, but just gassing the populace to the exclusion of armed manpower and armaments is very much a taboo. At least, legally and officially, although countries find ways to skirt that with defoliants, pyrophorics, nuclear weapons, etc.