Maybe because he didn’t have them? This was a guy who was resorting to having kids and the elderly fight near the end not to mention he was out of his mind not to mention he had no issue gassing to death women and kids in the gas chamber. No, I think if he had the opportunity to use something like Mustard gas against the allies he wouldn’t have hesitated for a second.
No, maybe as a last gasp perhaps. Hitler was exposed to mustard gas in WWI and knew the horrors of that. Also, chem weapons on the open battlefield is a logistic nightmare concerning various factors (Wind, lack of training, lack of supplies, over all environment, etc...). Also believe it or not, when it came to the battlefield, Hitler was all about following wartime conventions. This could of been strategic though because Hitler constantly accused the Soviets of breaking the ‘Convention for the Amelioration of the Condition of the Wounded and Sick in Armies in the Field’, ‘Convention relative to the Treatment of Prisoners of War’ and other Geneva protocols.
“Maybe because he didn’t have them?”
The Nazis had literally tons of tabun and mustard. They did not use them because they thought the allies had CW also and would retaliate in the cities.