cause it really wasn't a useful weapon.
they (and other nations at the same time) used it in WW1 mostly in an experimental setting. the ability to produce and deploy large quantities of chemical agents was relatively new and exciting, and they saw lots of soldiers sitting in deadlocked trenches at the front, so they figured that flooding the trenches with gas would be a good way to smoke them out (literally) to enable their own guys to storm and take over the enemy positions.
well it didn't really work out as planned. sure it killed and maimed a lot of soldiers, but it was super clunky and inaccurate, and very hard to control, with huge potential for friendly fire. also it wasn't nearly as effective at clearing trenches and dispersing the enemy as they had hoped, especially after the widespread adoption of gas masks, so the overall conclusion was that this shit just isn't worth the trouble.
now add to that the fact that ww2 battlefields weren't about massive numbers of infantry sitting in trenches, but about fast moving motorized forces and tanks with air support, plus lots of urban combat, and it becomes pretty obvious that gas attacks will be nearly useless. that's why it was abandoned and never picked up again on a large scale.