The Obama and McCain campaigns jointly negotiated a detailed secret contract dictating the terms of all the 2008 debates. This includes who gets to participate, as well as the topics raised during the debates. [Democracy Now! speaks] to Open Debates founder and executive director George Farah.
The Republican, Democratic parties, who exert near absolute control over these public forums, have determined and made sure that no third party voices are ever seen on the debate stage and can challenge their dominance of our political system.
[T]he contract[s] remains a secret, and the Commission on Presidential Debates, a private corporation created by the two major parties...
Unfortunately, much of the money that finances the presidential debates that are hosted by the Commission on Presidential Debates are private corporations that have regulatory interests before Congress.
http://www.democracynow.org/2008/10/2/no_debate_how_the_republican_and