I can think of many factors, none purely logical. Humans are not purely logical.
We are desensitized to "normal" deaths. Think of how many people die each year in car accidents, while odd and bizarre deaths make the news. Conventional weapons are, as the tin says, 'conventional.' These are the normal weapons of conflict, nothing special to see here, move along.
Chemical, biological, and atomic/nuclear weapons are uncommon, and easy to define as being different in kind, rather than size or effect. This bright line is easier to understand.
Of those three non-conventional kinds, chemical weapons were once widely used during WWI. The horror of chemical weapons is not so much the deaths but the large number of people who survived, and lived with afflictions for the rest of their life. While conventional weapons leave people with missing limbs and scars and mental problems, chemical weapons tended to leave people who needed much more medical care, and suffered more. Chemical weapons weren't part of WWII in part because of the memories of the afflicted of WWI.
But only a part. Another is that chemical weapons aren't that effective against trained soldiers, and a change in breeze can as easily send the gas against one's own soldiers, or to the civilian population, which increases the likelihood of poor collateral damage.
Thus, chemical weapons are more easily interpreted as a means of killing a civilian population, rather than killing opposing soldiers. This is not acceptable in our agreed upon fantasy of rules for good warfare.
Had it been conventional bombs we could continue with the belief that they were after enemy combatants, who used civilians as shields.