I'm assuming you're asking 'Why didn't the Nazis use chemical weapons in mass quantities in WWII on the eastern front?'

By WWII more powerful chemicals like nerve agents would have been used over chlorine gas (the Nazis had a large stockpile of sain gas). Also poisonous gas WAS used on the eastern front - but primarily to clear caves in the black sea area.

There are a number of reason why the Nazis did not use chemical weapons on mass scale on the eastern front, from the logistics of effective deployment (it would have stretched the already overtaxed luftwaffe), the threat to their own troops (and horses who did not have access to gas masks and made a significant proportion of their logistic backbone), to the change in warfare that made chemical deployment less useful (mobile vs static warfare), to the fact that the Nazis were looking to take over the east (cant have lebensraum if you cant use the land) and retribution (think an early version of MAD). Of these retribution seems to be the strongest - Hitler himself gave an order in '43 to not send nerve agents to the eastern front for fear that "local commanders would use them and trigger Allied chemical retaliation".

Sources:

Absolute War: Soviet Russia in the Second World War By Chris Bellamy

American Arsenal: A Century of Weapon Technology and Strategy by Patrick Coffey