You don't think that leaving a party out of the national discussion entirely is intentional? No, it's not a conspiracy. It comes from the fact that those parties don't tend to raise enough money to pay any of the major networks for much advertising.
I know it's just business... that's the way the system functions. If you advertise, you also get a little kickback in "news" stories, as well... you scratch my back, I'll scratch yours sort of a thing. That also makes the "grass roots" concept a bit of a thing of the past unless people wake the hell up. Money determines everything. Despite not having a "state press" per se, a press in a democratic society that is completely controlled by money to the exclusion of all other factors makes a country and a culture that is completely controlled by moneyed interests. If you don't think the content of your programming is strongly affected by the advertisers, you're just plain naive.
I'm not saying that the smaller parties don't regularly screw themselves over in the way that they function, mind you. Getting one person elected in one district (concentrated localized effort) would be a much better start than trying to run for president before having senators. I suspect that this problem is likely all about making money as well.
Basically, as long as the press continues to be monopolized, there's absolutely no chance for a new political organization to rise to legitimacy. The internet has the potential to change that somewhat, but in some ways it just makes it worse (targeted search results, "social networking" sites where people don't actually interact with anyone that wasn't already in their peer groups, mostly inaccurate and intentionally viral political garbage, etc.).
Seriously, dude. You really are being intentionally manipulated. No, it's not a "conspiracy" per se, since many of the players are competitors, but that's sort of what advertising, and by extension, everything that makes money from advertising (which would include the debates), is designed to do. The thing they all have in common is that they have enough money available to play the game.