Because its particles cannot move.

When water near the bottom of a vessel is heated, what becomes of it?

It rises to the surface.

What becomes of the cold water at the surface?

It falls to the bottom of the vessel, to take the place of the warm water.

How long will this circulation continue?

It will continue so long as heat is applied to the bottom of the vessel.

When heat is applied to the surface of the water, what is the result?

The water near the surface only is heated.

Why does it not heat all the water in the vessel?