Answer:
Styrofoam has a high resistance to heat transfer- that is heat does not "flow" through it very easily. Therefore, heat from the outside air does not enter the container very quickly. This happens because Styrofoam consists of expanded polystyrene. If you looked at Styrofoam under a very strong microscope you'd see millions of tiny pockets in the foam. These pockets trap stagnant air. Air (as long as it's not moving in a current) is very resistant to heat transfer. Hope this helps!