Biological has the advantage of not destroying the infrastructure of the place you are attacking nor making it inhabitable for thousands of years. So if you're wanting to take over the land after you remove the pesky opponents currently occupying it, nuclear is a really bad choice. Biological and chemical can be cleaned up and or inoculated against depending on method used.
The only reason nuclear weapons are easy to pinpoint is because so few actors are capable of making them. If you had one you could load it onto a semi truck, drive into the middle of New York or Moscow and detonate it, with all evidence conveniently destroyed in the blast.
Keep in mind that there are nuclear detection sensors deployed throughout the US and if you tried to roll a nuke-containing semi into New York, there would be a heavily-armed team trying to intercept you[1].
Supposedly[2], they are sensitive enough that it's untenable to transport enough lead around to shield it.
And in the short-term future, I think synthetic biology will represent an even greater threat than nuclear.
Why? Lethality, ease of manufacture once figured out, mishandling of process or materials, lack of regulation, ethnic/DNA targeting, etc.