A bipartisan organization of former Republican and Democratic party bosses called the Commission on Presidential Debates took over the organization and operation of the debates from the League of Women Voters in 1988. The commission secretly drafts contracts called Memoranda of Understanding that, according to the non-profit non-partisan organization OpenDebates.org, "dictate precisely how the debates will be structured… shielding the major party candidates from public criticism."
The commission facilitates corporate sponsorship of the debate, and neither the commission nor the corporations investing millions into the two major parties, are all that interested in allowing third party candidates access to this crucial forum.
The results, according to OpenDebate.org:
Candidates that voters want to see are often excluded; issues the American people want to hear about are often ignored; the debates have been turned into a series of glorified bipartisan news conferences, in which the candidates exchange memorized soundbites; and debate viewership has generally dropped, with twenty-five million fewer people watching the 2000 presidential debates than watching the 1992 presidential debates.
It's time to start agitating for change. As with most issues such as this, the beginning can be just a baby step. And maybe that step is found in this great post by Larry Lessig, who calls upon the political parties to make the presidential debates free for use and dissemination after their initial broadcast.