There's this military historian perspective [1] on why chemical weapons aren't used by regular militaries becase they aren't actually that effective (compared to spending the same effort on making normal explosives). It is slightly different topic to terrorist attacks but I think it touched on that as well.

Be warned, the blog is a complete rabbit hole of "oh god why am I reading analysis of logistics of Lord of the Ring armies at 3 AM".

[1]: https://acoup.blog/2020/03/20/collections-why-dont-we-use-ch...