Why are there only two Presidential candidates?

Why are there only two Presidential candidates? Professor Robert Schmuhl, who is Walter H. Annenberg-Edmund P. Joyce Chair in American Studies and Journalism
Director, John W. Gallivan Program in Journalism, Ethics and Democracy at the University of Norte Dame answers the question. He also gives a facinating insight into previous third party candidates. First in series of U.S. Embassy Dublin's Election 2012 videos.