Why just two political parties?

OK. The United States just had a downright nasty and also costly midterm election. Why do we have just two political parties? It has come down to an us vs them system, with the elected officials spending much of their terms trying to figure out how to beat the one realistic challenger and retain their office. Now that people have been elected, even if they actually come into office with good intentions, the party is thinking about how to remain in power. Would having more than two parties help to get the focus more on the reasons they were elected and the people that they were elected to govern, or would there just be more of a fight with more people fighting?