With how effective chemical weapons were in World War 1 why were they never used in the European theater of war? (Japan did use chemical weapons)

My theory is because war was a lot less static then it was in World War 1 so it was harder to use. But on a under equipped army (example early soviets on defense) it could spread utter hell through the lines.

Would love to see what the actual reason by this was and if I was correct.
~Cheers