One problem is that each state has its own laws as to how districts for the US House of Representatives are apportioned. Some require proportional representation, but most do not. They're re-drawn after every decennial census, and the Republicans did an amazing job of grabbing state legislatures in 2008 and 2010 to be in control when the new census came out. They gerrymandered the districts to such a ridiculous extent that even though the approval rating of Congress is at an all-time low, they're almost guaranteed reelection.

And I totally agree about the shame that we seem to be utterly locked in to a two-party system. I quite like a lot of what our Greens are for, but they have a zero chance of the presidency and not much higher for Congress in general. The two parties control the debate system and require a candidate to poll above X%, and because the Greens can't get any significant exposure, they can't poll high enough to be included in the discourse.