Here's why:

* Magnesium is highly reactive: It burns readily in air, combining with oxygen to form magnesium oxide (MgO).

* Carbon dioxide is a fire suppressant: It acts as a blanket, preventing oxygen from reaching the burning magnesium.

* No oxygen, no combustion: Since the combustion process requires oxygen, the absence of oxygen stops the burning reaction.

Important Note: While carbon dioxide is effective, you should never attempt to extinguish burning magnesium with water. Water reacts violently with burning magnesium, producing flammable hydrogen gas and potentially causing an explosion.