The only use of weapons of mass destruction by the US was in WWII, and nukes weren't consider off limits yet. Since then we helped to get them banned in as many places as possible, and in general find them too indiscriminate.
Just like the world community agreed chemicals were a bad idea after WWI. Countries using the WMDs use them once in one war and found them so reprehensible that everyone gets together afterward and an agrees that these are too much even in war against hated enemies.
Anyone using any WMD after the hard lessons about them have already been learned is so evil that it is trivially easy to get many armed nations, even those with only the loosest ties, to attack them.
Consider: https://en.wikipedia.org/wiki/List_of_weapons_of_mass_destru...