The answer you're probably being asked for is because it reflects heat back in rather than radiating it out.
But (and this is a big but) it's metal which makes it a very good conductor of heat so although this is a good design from the point of view of radiatio it's a bad design from the point of view of conduction .
As Naz says teapots are often made of china even though metal is cheap and easy to make - China conducts very little heat and so is preferable to metal from a heat loss point of view.
However a metal teapot wont chip or break easily which is the real reason metal teapots are used - especially in cafes or restaurants