This is just a guess but Assad won plus he had the backing of the Russians why would he use those weapons in Syria. It does not make sense for him to do it. He already won.

Second question where did those weapons come from in the first place. Syrian scientist developed weapons of mass destruction? Remember I said the Russians could have taken them with them when they left Iraq just before we went to war with Iraq. I think Isis had them and just used them on civilians. I think trump was played by his military. Well to late now to change anything.