You'll find the answer to your question in these two answers from the FAQ:
To summarize: After WWI all sides felt they needed to rein in the unmitigated use of chemical weapons for humanitarian reasons. The Germans also felt rightly that if they used chemical weapons then the allies would use chemical weapons on them in turn: mutually assured destruction. After the allies gained air superiority it would have been suicide for the Germans to use them as now the allies could do the same, but from the air.
As to why they used them in concentration camps and not the front, the treaty banning chemical weapons had a corollary loophole for "lesser races," which for the Germans certainly included the Jews and other unwanted peoples.
To add onto this would the allies use poison gas back? More as a defense?