Why nuclear? Why not chemical or biological WMD?

Chemical and biological really should not be grouped into the WMD label. They are not nearly as effective in the M and the D. See the saran attack on Tokyo. One small nuke would have been thousands of times more deadly. For a chemical attack to be as effective as a nuke one would need hundreds or thousands of bombs dropped on a single city. If you are going to do that then a firebombing raid can be much more effective for destruction. 300 B-29 caused more damage in a single night over Tokyo than either of the nuclear attacks later in the war (modern nukes are much bigger though).

Because nuclear - with exception of neutron bombs - destroys buildings, resources and materiel as well as people; chemical and biological attacks mostly don't.