Really, chem is hard to disperse over a large area and still be effective. You might get a subway car or two with chemical weapons, or maybe a block or two of people on their way home.
Bio's a little bit more insidous, but it's still counterable at some level via antibiotics, vaccines, and isolation of the infected populations.
Nukes, OTOH, leave a crater about a half-mile across, and one in a major city, well, let's just say the insurance underwriters wouldn't be too happy about that.
Not to mention the 1+ million people that got incinerated inside of 5 seconds. Nope, none of them are going to be much of anything any more, other than dust.