As I said in another thread....
It is all about the size of the theatre....
Look, chemicals just aren't all that useful as area denial weapons in a theatre as big as the Eastern Front. Yes, there are a few choke points here and there, but for the most part, you don't get continuous lines of resistance to any significant degree, and anything you contaminate (and by the way, Tabun - while significantly deadlier - isn't a particularly superior area denial weapon in terms of persistence to thinks like mustard gas) will be eaily routed around. As a secondary point, both sides have very little (any?) resistance to using slave labor for cleanup/decontamination work, not to mention basic recon operations, which means that the chemicals you do use are going to be cleared up fairly quickly.
Chemicals made some sense on the Western Front because of the compact nature of the front and the very, very high troop densities. Neither of these are present on the Eastern front to any significant degree. The Germans will have marginally better chemicals, the Soviets will have better delivery systems and substantially better industrial and logistical infrastructures to deliver the weapons. With this in mind, I rather doubt that the Germans are going to see this as much of an advantage for them, and the Soviets are simply not going to need them.