Why do minor political parties in the US receive so few votes?

Another reason for this might be that many countries require all citizens to vote on election day. The US does not. Thus, an unmotivated voter in the US stays home whereas in other countries he might pull the lever for some random fellow.

CGP Grey has a nice video explaining why the first-past-the-post voting system usually ends up with a two-party system. As others have said it burns down to:

You hurt the party you can most agree with by voting for a minority party (which doesn't get elected anyway). So you vote "strategically" for a party with a good chance of winning.

Not an answer that is going to be upvoted, but most people in the USA, and other single-member constituency systems, vote primarily against a candidate, not for one. (I have no evidence of this, except personal observation.) This is why, particularly in the United States, politics can get very nasty and personal, and the most effective political advertising is negative. People don't vote to get their candidate elected, they vote to keep the opposing candidate out.

Some countries, like Russia, as I remember being told once, have a minimum percentage that a winning candidate has to reach. This is not the case in the USA (or Britain). In Australia, they have a single transferable vote, but single member constituencies. Australians have to rank all the candidates in their preferred order. Except for a single regional party, all constituencies end up being a competition between the two largest parties.

It's the same in almost all countries with similar electoral systems. Britain, Australia, Canada, New Zealand, South Africa, and historically in many South American countries. A partial exception is India, where some parties have regional support, which can make for a third force.

The trend is exacerbated in the USA, where primary elections give the voters a more direct voice in choosing the two main candidates, so that some of the differences of opinion that lead to third parties elsewhere are sorted out at that stage. In this respect, the formal actual election in the USA is rather like the run-off election between the top two candidates you get in other places, such as French presidential elections.

Another reason that minor political parties receive so few votes in the United States is the strength of party identification. The reality is that most people take on the political party of their family, and once they are a "member" of a political party they are very unlikely to change that membership for a minor political party.

The book Partisan Hearts and Minds talks about how political party affiliation is similar to religious affiliation, except that you are more likely to switch your religious affiliation than your party affiliation. This strong sense of party identity is paired with a general disinterest in the specifics of the political system which has been expertly showcased in the seminal work American Voter, and the horribly depressing, but spot on, book Stealth Democracy.

So, basically you have a system that was not designed with political parties in mind, populated by voters that are born into their political camps and have little interest in the specifics of the government/political system, which means that they are not terribly inclined to go through all the additional work that would come with supporting some third-party candidate.

In the United States, most voters vote "strategically." That is, they may favor a third party candidate during the campaign, but when election day rolls around, they will vote for one of the major parties in order to not "waste" their vote.

For instance, in 2000, Ralph Nader ended up with 2% of the vote. Most of his "natural" supporters ended up voting for Al Gore, and the ones who didn't probably wish they had, because those people would much have preferred Gore over Bush.

In 1980 John Anderson was the preferred choice of 20% of the voters, but only 10% (an unusually large number) voted for him that couldn't stand either Reagan or Carter. But many of his supporters backed "the lesser of two evils."

I heard an interesting answer to this question a while back, and while it's probably not the whole answer (it could go hand in hand with some of the others to this question), I think it puts an interesting perspective on things. It goes something like this…

In other countries without a two-party system, a parliamentary election happens and four, five, six or more parties get voted in in significant numbers. They then get together in meeting rooms and build coalitions so they can get things done (or block the other coalition(s) from getting their things done), so what you end up with is parties that sometimes have very different goals building coalitions together.

The two parties in the US, on the other hand, are already quite broad coalitions. The Republicans includes the Religious Right who want to see more influence on policy in accordance with Christian principles, and libertarians who are skeptical of religious entanglement in government. The Republicans also include the Tea Party folks, who want to see a reduction in government spending and control, and President George W Bush, who could only be called a fiscal conservative in relative terms (relative to the current President, he was fiscally conservative, but…).

The Democrats include peaceniks skeptical of US support of Israel, as well as a majority of American Jews. The Democrats also tend to get votes from those in favor of government recognizance of gay civil unions and marriages, as well as many southern and black Baptists, who as a rule are very much against gay marriage (it's speculated that one of the reasons that California's Proposition 8, which would have made gay marriage legal in the very socially liberal state, ultimately failed was because of the strong turnout by black voters in that election, which also elected President Obama) (I redact that last parenthetical for reasons mentioned in the comments of this post).

So if other countries have elections, and then build coalitions, it could be said that we Americans build coalitions in the form of our two parties, and then have elections.