the simple fact of the matter is that there is strength in numbers.
The more that you divide based on your beliefs the weaker that you are.
A great example would be the election of 1844, when Henry Clay (remember him- he is one of the most important people to know in US history) ran against James K. Polk for the presidency.
Polk barely won, mostly because he managed to get NY's electoral votes. He won by about 5k votes. The liberty party, which was closer in political views to Clay's Whig party than to Polk's democrat party, got about 15k votes. If they had not run their candidate there is a good chance that Henry Clay would have been president, and while he was not exactly aligned with their views, he was still closer than Polk.
Essentially, people would rather have someone that they agree with on most issues than someone that they disagree with on most issues, so they are willing to compromise and thus the USA has the two party system.
Hope this helps