Chemical and biological weapons aren't easy to deliver, as discussed in this article. There are several likely reasons they have not been used so far, including:

1. The weather hasn't been favorable. Chemical weapons are dispersed by the wind. They can even come back on you if the wind shifts. During WWI, chemical weapons ended up killing about as many soldiers on the side that launched them as they did on the enemy.

2. The US forces may be too dispersed for an effective attack.

3. There may be political factors as well, such as turning world opinion against them. Don't expect them to give a damn about that as the noose tightens, but delay actually supports their propaganda efforts at the moment.