Why Polar Bears cannot survive in a hot area?

The body of Polar bears is quite adapted to survive in extremely cold weather. That's why they cannot survive in warm weather.

Even in Arctic chilling weather, they get overheated sometimes, if they run for a few minutes continuously while chasing their prey.

So, in most cases, they have to give up on their prey, as the heat inside their body gets entrapped due to having a thick layer of fat and fir.

 Their primary preys are seals which can barely survive in warm weather just like polar bears. Also, to maintain the food-chain in the polar region they need to stay there.