Why does the United States only have two political parties?

Right now, in America, the country is close to evenly divided among R and D's and we see what is happening with the budget, and other issues. Can you imagine how much more harmful, and unproductive it would be having the country split between 3 or 4 different political parties. You may like that sort of thing in your part of the world, but that is also why your part of the world is in such disorder.