For far too long in this country, politics (and debates in general) has been run by the two major parties – Democrats and Republicans. While both of these parties are close to the center of the political spectrum and represent a lot of people, there are others out there with other ideas. Yes, we have third parties, but they haven't won an election (or even been competitive) in decades. Part of the reason they aren't more competitive is that no one knows about them. People in this country have grown up know Republicans and Democrats.. and if you're not one of those two, you don't really matter. If you want to vote, you pretty much have to choose the lesser of two evils.
Also, third parties are very hard to get started. The third party laws in many states are so insane they are nearly impossible (but that's a different issue). If third parties were allowed to contribute to televised national debates, awareness would drastically increase. Instead of being forced to choose the lesser of two evils, people would actually have a broadly publicized candidate for whom to vote.
In the end, the issue comes down to equality. We say we're ALL equal, but then we turn around and say if you're not a Democrat or Republican, you're screwed. Open up the process and make it easier for third parties to participate!