Q: Why are there only two major parties in the U.S.?
A: The winner-take-all system in the U.S. favors two stable parties.
It seems as though there should be one in the middle, with Democratic views on the economy but Republican views on social issues, or vice versa. Or do the two parties really represent the current divide in the U.S. population?
The U.S. political system is based on what political scientists call a single-member district plurality (SMDP). That’s a fancy way of saying that the U.S. elects representatives from particular districts, with the person who gets the most votes in a district (also called a plurality) winning the seat. Each district is winner-take-all, and votes in one district have no effect on other districts. Presidential elections, though nationwide contests, are likewise really state-by-state races, thanks to the Electoral College, in which every state except Maine and Nebraska awards all of its electoral votes to whichever candidate wins a plurality of the state’s votes.
In the 1950s, the French sociologist Maurice Duverger observed that stable two-party systems often develop spontaneously in places that use single-member district pluralities. Political scientists now refer to this tendency as "Duverger’s Law."
The reasons here are mainly statistical. Third parties may have statistically significant support (maybe 15 percent of voters in every district supports a third party). But in an SMDP system, the third party may well not win any seats. So those voters will likely join with another party and look for a compromise candidate that could represent them. Similarly, suppose that a district has 200,000 conservative voters and 110,000 liberal voters. One would expect a conservative candidate to be elected. But if two conservative parties each run a candidate, then a liberal candidate may well be elected – unless the conservative parties unite behind a single candidate.
– Joe Miller
Duverger, Maurice. "Party Politics and Pressure Groups." New York: Thomas Nelson & Sons, 1972