I think pohnpei's response is right on. Ours has been designed as a two party system, first by the Founding Fathers in order to avoid what Washington called "The dangers of faction". Since that time, the two party system supports and continues itself, making it very difficult for other interested parties to enter the race.
We should acknowledge that there are many more than two parties in the US today. The Green Party, Socialist Party, Communist Party and Reform Party to name a few. These parties always put up candidates for the Presidency, and always lose badly. They are not even allowed to attend debates.
The Democrat and Republican parties have loads of cash, name recognition, a base of support, and a hammerlock on power. They also try to absorb other political movements and would-be parties by including some of the issues important to them in their party "platforms" adopted at the national conventions.