Why are some types of fire extinguishers cannot distinguish a magnesium fire?

A magnisium fire burns at a significantly higher temperature than a typical fire with other combustable materials. If a water extinguisher were to be used on a metal fire, the heat of the fire will cause the water to break down into its chemical constituants Hydrogen, and Oxygen (H2O), and instead of removing heat from the fire it will actually cause the fire to intensify due to the addition of fuel Hydrogen (H2) and an oxidizer Oxygen (O).

Other extinguishers can extinguish the fire, but again due to the extreme heat it will take significantly more agent to extinguish this type of fire than would be needed for a standard combustables fire. An extinguishing agent which is designed for use on a metal fire can withstand the extreme heat and will extinguish the fire by blanketing the burning metal and smothering the fire by depriving it of oxygen. Dry Sand for example would work well. There are other agents which could be used on this type of fire which extinguish the fire interfering with the fire's chemical reaction rather than smothering it.