There was no good reason to use it. It's not generally effective in gaining advantage if your opponent is equipped with relatively basic gas protection equipment and training, and everyone was by ww2. It's main effect is in slowing everything down on both sides, and when your war plan is predicated upon rapid movement it's simply the wrong tool. Sure, a surprise release of heretofore unknown nerve gasses probably would've stopped D-Day cold, but three to six months later the very same stuff would've been raining down on German cities.
Then there's the idea that Hitler expected us to have the same nerve gasses he was producing... The particulars that led to the big "a-ha!" were published in international scientific journal articles. The pesticide work that led to it was being jointly conducted here and there. Dupont and IG Farben were the two biggest dogs on the block and they played very nicely together. Hitler also knew of at least some of the goings on at Porton Down and Dietrich. He "knew" (with some questionable assumptions regarding the seriousness and success) that both the US and England were preparing for chemical warfare, and that the production capacity of the US was quite capable of outpacing even a peacetime Germany. He also had a far more important mission for the IG, synthetics to replace the oil, rubber and nitrate supply cut off by the blockade.
Then there's something else I read recently... It would've killed the horses. The wermacht was overwhelmingly horse powered. They had masks for the horses but they didn't pass enough air to let them work while wearing them, and masks alone only work for those gasses that require inhalation. They surely had not worked out a suit that could've protected horses from agents that would affect the skin.
I think people lean way too hard on the "Hitler was gassed in ww1" bit, but I think that has a lot to do with overestimating chemical warfare in general.