While Hitler was quite traumatized by gas attacks, a more practical reason is that England had about 4 times the amount of nerve gas ready to dump on Germany.
Go back to the pasture, Beef-boy! 
England had no nerve gas in WW2.
Nerve gas was invented by the Nazis. There were no neurotoxin gasses available outside of Germany.
The UK had old gases, like cyanogen or Mustard Gas, and it had an (completely secret, the Germans knew nothing about it) active offensive biowarfare program, but nerve gas was unknown to the Allies, until the gas supplies were captured at war’s end.
Ah. Well, they had several times the amount of poison gas, in general, then, than the Germans did.