They are specially designed to limit the amount of heat conduction between the liquid and the surroundings. Air is a very good insulator, better still is a vacuum. By having a smaller vessel in a larger vessel, an airspace or a vacuum can be between the smaller vessel within the flask and the outer vessel. The thermos flask also limits the amount of conduction through the solid part buy limiting the contact points in the design. Some flasks also have a silvery surface to limit transfer of heat through radiation.