Biological weapons also have the distinct problem of control: you can release them, but you can't stop them afterward. So, you need to be able to keep your own

Biological weapons also have the distinct problem of control: you can release them, but you can't stop them afterward. So, you need to be able to keep your own people safe from the release, which means expensive vaccinations across your population. And how do you test that your vaccination works? Or that it'll still work after your bioweapon mutates? Bioweapons are useless unless your goal is to kill off everyone.