Everything is geared to a two-party system. There are actually many political parties out there, but their membership is so small that they seldom get anyone on the ballot and when they do, there is usually no money to campaign. I was covering a political forum for a U.S. Senate race once and one individual, representing a party I never heard of, recommended the use of low yield nuclear weapons, whatever those may be, to stop a conflict.
I think he got two votes in the general election. People want their vote to count and they see the two parties as the best way of doing that. We did see when Ross Perot launched his third party campaign that he split and divided the conservative vote, making it easier for Bill Clinton to defeat the incumbent George W.H. Bush. In that election, Clinton got 43.9 percent of the popular vote. Bush got 37.5 and Perot got 18.9 percent. If you assume that all of the Perot vote had gone to Bush, then he would had probably been elected president, depending on the Electoral College vote. It is possible that Bush could had picked up even more votes, since many conservatives were confused over who to vote for, Perot or Bush and simply did not vote. This is all speculation, but is is a possible illustration as to how a third party can work to the advantage of one of the existing major parties.