The main reason is that we are afraid of American retaliation!

Although the United States started its research on chemical (and bacterial) weapons slightly later than other Western powers, during World War II, they already had specialized institutions and bases to conduct research in this area. For example, the person in charge of the so-called "Fort Detrick Project" is named Ella Baldwin. The then President of the United States, Roosevelt, even said: "This project (that Ira is responsible for) is more devastating and more promising than the atomic bomb project that Oppenheimer was responsible for..."

It can be seen from this that , if the United States was attacked by Japan's chemical (and bacterial) weapons at that time, the consequences would inevitably be to immediately adopt the same means to "retaliate" against Japan. But it’s hard to say whether Japan is fully prepared in this regard!