That's a pretty easy question to answer. He used sarin in 2017 and only had an air base blown up. That set a precedent that he's not going to really face much worse. Regardless of the recent Douma incident let's say for the sake of argument he, without doubt, did use them, his punishment was only a few blown up warehouses. Yes he may have already won the war but given the very minor price he's had to pay he has absolutely no reason not to use them to end the war more swiftly.

More importantly once this bloody business is all said and done who would ever want to again go up against a man who used chemical weapons in the past with virtual impunity? His using them now is much a deterrent for future civil wars as it is a means of ending the current one more quickly.