Numerous factors encourage a binary party choice in the United States as opposed to the multiparty system that exists in other countries. An important structural reason that the United States has only two viable parties is our “winner takes all” elections, as opposed to the proportional representation common in parliamentary systems.
In US elections, the person who wins fifty percent plus one of the votes is elected in the district. The person or party who comes in second or third gets no representation. This “first past the post” method of having elections exists at all levels of representation: congressional districts, statewide elections, even the presidential election is based on an electoral college system in which only one person wins all the votes for a particular state. In other democracies, elections take place nationwide, with parties who get above a determined threshold of votes receiving at least some representation in the legislature.
Therefore, to be a viable party under a proportional representation system it is not necessary to win 50%+1 of any particular district, but rather a small percentage of the popular vote. Proportional representation leads to a greater diversity of parties since smaller voices are still heard. Another reason why two parties dominate the US political landscape is that they are constantly in power, giving them the ability to entrench their positions and prevent any change that would weaken their grip.
For example, many congressional districts are gerrymandered in such a way that one party has an overwhelming majority over the other party. Since there is such overwhelming political force, it is hard for the second party, much less a third or a fourth to apply much electoral pressure. The two major parties tolerate this practice because it entrenches their respective candidates, meaning they have to expend less energy in order to get them elected. Another example of entrenchment is the lack of third-party candidates at Presidential debates.
Many voters use debates in order to develop their opinions and make choices about who they want to vote for. Without any third-party candidates represented, the voters may not realize that they have that choice and may not be able to form an educated opinion on the lesser-known candidates.
Furthermore, the US population traditionally identifies with the two major parties and uses party labels as cues to determine how they should vote. Although in recent years there has been a decrease in party identification (or dealignment) with more independents emerging, a substantial amount of people still classify themselves as either a Democrat or a Republican.
Since many of these individuals grew up within that framework, it would be difficult for them to throw those classifications out the window for a third party. Part of the reason that people identify with the parties is that they use the labels as cues on how to vote. In other words, they may not need to know everything about the candidate’s positions, but only his binary political party in order to make a choice.