And the reason was...
Both sides had chemical weapons in WWII, but even Hitler didn't use them. However, the Japanese did use chemical and biological weapons against the Chinese. If this is true, the British used botulism on at least one occassion.
The reason Hitler didn't use his chemical weapons against the Allies was that he knew he'd be hit by them in return.
Likewise, in the 1991 Gulf War, Saddam was told in no uncertain terms that any use of chemical weapons would be met with harsh retailiation, perhaps with nuclear weapons. They didn't rule that out last year, either. Saddam was nuts, but he wasn't that stupid.