Because chemical weapons are brutal and efficient. You get all of the kills of a major conflict without the casualties. There's a reason such weapons are banned under the Geneva Convention; they work quite well on a physical and psychological level without the need to risk lives of the person dropping the bombs or lobbing the shells.
They work so well in fact, that now instead of just winning his war, his skies will be host to drones and the rebels will become the new Syrian regime.
Wow, that was a very effective tactic by Mr Assad, obviously his goal was simply to give Merica a reason to bomb the shit out of Syria.