It hasn't only happened twice. It's happened at least 40 times according to Wiki and the overwhelming majority of these chemical attacks were against Assad's enemies.  Some claim the number of attacks is as high as 160. This attack made the news since it's caused the most casualties since the Ghouta chemical attack  in 2013.

Furthermore, you blow up sarin or chlorine, you destroy it. An airstrike against a chemical weapons facility using explosives would not have caused this sort of event. Chemical weapons are also extremely difficult to control so they can only be used under certain conditions. The First World War taught everyone that lesson. The airfield where the airplane took off and landed is in desert probably for that reason.