Why are there just two parties in America?

In the United States, there currently are two major parties including the Democrats and the Republicans. These two parties are sometimes at odds with each other. However, in the past there were other parties including the Whig Party. Today, there are other smaller political parties, but the Democrats and Republicans represent the vast majority of the citizens and those in Congress.

Some of the smaller parties include today the Libertarian Party, Modern Whig Party and the Socialist Labor Party. A few years ago, there was the Tea Party but over the past few years, it dissolved. These two parties, the Democrats and the Republicans, have been around for so long that it seems to be difficult to change them so that they are weaker.