There are a number of reasons.
First is that there is no benefit to a third party because of the way Congress works. In a parliamentary system, coalition building is the norm because those coalitions can create an executive if they're powerful enough to control parliament. You cannot be top dog no matter how much of congress your party controls. If the Republicans had all the seats in the House and Senate, a Democrat could still be President and therefore control the executive. So in federal elections, the chances of getting your party in as President only matters in presidential elections.
This is further reinforced by the electoral college system. In that system, each state gets a certain number of people who are electors. The number of electors a state gets is based upon the number of Senators and Representatives they have in Congress. That means every state gets at least three electors. There is no connection between electors and Congressional Representatives or Senators, it's just a way of figuring the number of electors per state. What matters is who the people of a state votes for as electors. When Americans vote for President, they're actually voting for electors. If you pull a lever for McCain, then you're voting for state electors who are pledged to vote for McCain in the Electoral College. If you pull a lever for Obama, then you're voting to send someone pledged to vote for Obama to the Electoral College instead. While Electors are not legally bound to vote for the candidate they're pledged to vote for (amazingly), they usually do.
What kills this system for third party candidates in Presidential elections, is that the states themselves can decide how votes are applied to electors. 48 of the 50 states and the District of Columbia have a, "winner takes all," system. So if a state with say, 10 electors has a 51%/49% vote between two candidates, all 10 electoral votes go to that 51% winner. Once again, as with lesser elections, the loser gets nothing. The US has a "first past the post," election system. Whomever gets the majority of votes, no matter how small, wins. So let's say there are five parties vying for one seat. If the votes are evenly spread, even two or three or even one vote pushes the candidate of one party into office even if they only received 21% of the vote.
The second is that we have a single member voting system. Under that system, local and regional elections are rewarded with only one person being elected to office. Under plurality systems, even small parties have a chance of getting a few seats in various types of government even if they don't win a majority of votes. In the US, if your party doesn't win the whole election, you get no seats and no power.
The third reason is that historically, third parties that get popular tend to have their planks co-opted by the larger parties in an effort to stifle third party popularity.
In the US system, to get representation, you stand a much better chance of doing so if you belong to one of the two major parties. There are third parties, yet only one state, Vermont, manages to vote for non-major party members in big elections. Vermont is tiny and has two Senators (as all states only get two) and only one Representative in the House (every state must have at least one). One Senator is a Democrat, but the other, Bernard (Bernie) Sanders, is a Socialist. The term, "socialist," is so loaded with Cold War overtones of Communism, that he instead describes himself as, "Independent," for the sake of his ability to do business with the Democratic party with which he is aligned. The Democratic party never uses the term, "socialist," and if they were to allow Bernie to join in coalition with them, then the right wing would be all over them for associating with a Socialist. That's how tough it is to be an outside candidate in this system.
As the world having to thank the US for democracy.... uh no. Ancient Greece pretty much invented democracy and modern democracy is universally acclaimed to have started with Magna Carta, gradually developing into parliamentary democracy. Other countries such as The Netherlands, Iceland, Switzerland, and San Marino have been democratic longer than the US has even been around. Yes there were restrictions, occasional setbacks, and revisions, but do recall that when the US was founded, the poll tax limited voting to the landed and merchant classes without giving suffrage to slaves or women or the poor.
American democracy, as it is writ, is not terribly democractic. The system I described above illustrates why. Parties without the majority in any election get no representation. In many parliamentary democracies, this isn't the case at all. Small parties that represent particular interests can usually get a few seats in parliament and even gain voice far beyond their numbers if the members of that party gain concessions from the larger parties in exhange for creating coalitions. In these parliamentary systems, more voices are actually acknowledged and given a modicum of power than in the American system.
