What are the advantages of Chemical Oxygen Generation over Gaseous Canisters on board of aircrafts?
Pressurized oxygen tanks have two big minus points: they are heavy, and they are essentially a small bomb. But they have two big plus points: they can be refilled easily and they don't generate any heat when you use them.

Chemical oxygen generator plus points are they are lightweight, self-contained and more-or-less maintenance-free (they don't leak). Minus points are they are single-use, time-limited and cannot be shut off.

As nearly everything in aviation is a balance of good and bad, we often see both systems being used at the same time. The flight crew will use oxygen on a regular basis - sometimes procedurally, sometimes just to clear up a headache. They need to be able to turn it on and off, and the airline doesn't want to replace the system each time.

Passengers rarely need oxygen, and when they do it's only until the plane descends to breathable altitudes (which will happen really quickly).