Probably because this:
Made Assad test the waters and see if the US would react to a chemical attack.
I have no doubt Assad use chemical weapons if he deemed it strategically beneficial, but I refuse to believe that he's so stupid that he would have made such a miscalculation like the one you describe.
Miscalculation? He's only had a runway damaged and a few men die. Assad wanted to test to see if Trump was indeed letting Syria go, and it turned out, Trump wasn't.
He still has the support of Russia and a strong hand strategically. Nothing has changed.