Preface: please forgive typos, as it is well past happy hour and I'm on a tablet.

Hitler didn't use chemical weapons because he didn't want chemical weapons to be used against Germany. It's a very old concept called 'reprisals,' which basically means whatever the other side does against me, I can do to him and worse as punishment. Think of two boxers: they start off with all the honors and rules of boxing including not hitting below the belt. When Boxer 1 punches Boxer 2 below the belt, Boxer 1 should expect a reciprocal upper cut to the lower regions.

When WWII started, international law wasn't perfect. It still isn't perfect. But, the consensus on chemical weapons was generally agreed upon by world leaders: don't do it. (This is what I amstudying right now: the difference between international law/treaties and how leaders actually respected/honored them in WWII)

Churchill even wanted to use chemical weapons against German civilian targets as a reprisal for the V2 rocket attacks, but his staff convinced himnot to(because it wasn't necessary to achieve their strategic goals, and because Hitler threatened to gas English civilian targets)

Hitler spoke publicly on these terms: if his enemies use chemical weapons, then he will unleash a fury of chemical weapons. But, he wS only going to use them if someone used them against him.

Edit: also, efficacy comes into play. Gas, after Ypres in WWI, wasn't all that effective on the batllefield. Had Hitler used gas against the Soviet lines, it may not have budgex them much. So, he had to weigh the minimal gains on the front line vs. the massive German casualties that would result from Allied gas attacks against German cities.