Because it's hot there . So they can only handle the cold like other Arctic animals.
Because polar bears are right at home in the watery polar regions of the northern hemisphere. They are at home in the water because their body fat and dense, oily fur help to make them buoyant, as well as protect them from frigid temperatures at sea and on land. Even the hair of their coat is believed to have the effect of channeling ultraviolet light to the surface of their skin. This helps to keep them warm. No dry deserts for them!