Answer: Because you want to get the job done fast.
Nerve agents and other battlefield chemicals are an area weapon. Be it town or trench where your enemy is hiding, your deadly gas will find them.
From the waterlogged northern European plains of World War I to the arid mountains and marshes of the Iran-Iraq conflict, chemical weapons have been used when armies get bogged down and commanders get frustrated.
When the urge to win outweighs warfare's cold calculus of routine slaughter and indignity, the danger is a slide into the depravity of craven criminality.
Adolf Hitler used them in his extermination camps, diabolical in his annihilation of his enemies. So when I heard from two separate sources close to Syrian President Bashar al-Assad's thinking that he planned to retake the country's largest city Aleppo in a "swift battle of high causalities" so he could go to peace talks in Geneva with "facts on the ground" in his favor, the use of chemicals in Damascus' suburbs against resurgent rebels made sense.
Criminal Assad know how effective it is… he will use it again again…why shouldn't’ he? Job done easily…. and cheap economical... Russia knows very well...