Because the type of chemical weapons that were used in WWI were not really very effective then and would have been completely useless in WWII. When gas based chemical weapons were used they often were nullified by the wind or even turned back on the user. During the First World War the armies were bogged down and stationary but during the Second they were far more mobile and often operated in smaller groups which would have made those kinds of weapons impractical. Since then chemical weapons have mostly been used as instruments of terror against civilian groups.