Because it's a weather effect, not a terrain effect. If you're near a volcano the terrain itself is volatile and causes fire damage. If you're in a poison swamp the terrain is poisonous and spreading gas around.

Sandstorms in a desert are just something that might happen in a desert now and then because the wind decided it was time to play. Same with blizzards. Terrain effects are inherent to the terrain of the map itself and never stops existing.