The international ban on chemical weapons, as well as the almost universal revulsion of such, was born of the trenches in WWI. Not only was this horrifying to the press, but it was the first time you had mass casualties and long term survivors. It was the survivors that instilled the most horror. Prior in history, those that came in contact with chemicals died…or the environment so weakened the concoctions that there was no harm inflicted.
The natural environment of the middle east combat zones are not conducive to effective utilization of chemical weapons. The most effective aspect of chemical weapons use in that region is the legacy of fear their use instills.
The real fear is when the bad guys employ chems in an environment where they CAN be more effective than artillery or bombardment.