The purpose of a circus is to make money, not to make sure the animals are provided with natural conditions or even the right kind of food. Animals which are designed to roam the plains or the jungle are regularly confined to small, cramped quarters.
Whenever you see an animal do an amazing feat--such as jumping through a flaming hoop--consider that this behavior goes against every natural instinct in the animal. Usually the way they are trained to do such a trick is to use pain to gain their cooperation.
According to the Humane Society of the United States, many animals are not even a permanent part of the circus, but are leased from animal brokers. They are shipped from location to location, with little regard for their welfare.
When an animal has outlived its usefulness to the circus, they are usually sold to zoos, game farms or even to research labs.
If you are looking for entertainment in a circus environment that does not abuse or exploit animals, consider going to a "human" circus, such as Cirque du Soleil, which only uses human performers.