[seabass-labrax]

It's because of the direct effects; chlorine gas for instance will almost instantly blind anyone exposed to it, and tear gas can also be fatal. My great-grandfather was gassed in the First World War and only narrowly survived. Chemical weapons were technically already banned by this point, but it was WW1 that prompted the modern Geneva Protocol (not the Geneva Conventions; these are slight different). Unfortunately, none of the Geneva treaties cover their use outside of wartime.

[pclmulqdq]

Many people don't realize this but many of the banned weapons are ones with a decent chance of maiming people but leaving them alive. This is worse for everyone involved in a war (everyone here meaning politicians and generals, not soldiers) than killing them. Cluster munitions, flamethrowers, and anti-personnel mines all fit in this category, too.