I guess that’s because they’d be revealed as the ones using them?
It’s probably one of the most effective weapons they have, because not only does it kill a lot of people with little manpower (and if the guy doing the gassing gets gassed himself, always a danger, heck, he’s going off to get his virgins), but the ability to blame it on others and get some good propaganda photos is priceless.
Does anybody remember about three months ago when the “rebels” were attacking a remote facility in Syria that the army was heavily defending - because it supposedly had the gas stocks that Assad had gotten from Saddam? This was something that couldn’t be discussed here, of course, because it was always the liberal mantra that Saddam had no chemical arms, although it was well known and even demonstrated that he did. It was also one of the reasons the US considered going into Syria during the Iraq war.
IIRC, the facility fell although the government had managed to destroy most of what was there first. So the “rebels” would have gotten something, although they may not have much, but even in its probably degraded condition, it was sufficient for an attack or two. And if Assad is even seen to be giving up his chemical supplies, this is going to crimp the “rebel” style.
My guess is that a lot of the gas that is probably still in the same barrels that are 10,15,20 years old is starting to leak and may be unstable as well as useless. It has to cost quite a bit to maintain such an arsenal that has low pr value when used and is probably a nightmare to move.
Getting rid of the gas is probably a big plus for Assad. It may take the other terror types a while to figure that one out.