Chemical weapons were easily banned because they are almost entirely useless as long as you have access to conventional weapons. They are expensive to store, very inefficient for actually killing the opposition, easy to guard against, have a higher chance of turning back on you, and will slow your advance if you use them in an attack. They are at best a tool for terror or desperation.
Think about the Tokyo metro attacks and, not to minimize the tragedy, but how many more lives would have been lost if they had detonated similar quantities of explosives instead of spreading sarin gas - and that is inside a closed tube, with no protection whatsoever and with most victims receiving no medical care until many hours later, as hospitals were entirely unprepared for a gas attack.