I'm not saying that a chemical attack would be a completely trivial matter, but it would almost always be preferable to being hit by the same weight of high explosives.
So, if your aim is to kill and injure as many people as possible, you'd be a fool to use chemicals. And yet chemicals are rated as WMDs, while ordinary explosives aren't. So too are biologicals, even more amazingly. Biological "weapons", in the modern sense, have yet to be even demonstrated.