WHY THE PEOPLE OF THE USA CHOICE ONLY BETWEEN DEMOCRATS AND REPUBLICANS PARTIES?

We have seen other parties in the USA like Green one but when the most people tired from donkeys they vote to elephants and vice versa!!! The most of people in the world think where the USA only has two parties because no one understand this!
WHY THE PEOPLE OF THE USA CHOICE ONLY BETWEEN DEMOCRATS AND REPUBLICANS PARTIES?
Post Opinion