An interesting article in the Washington Post referenced below states that:
The U.S. picks its elected officials in a way that creates a two-party system
Duverger’s Law says that the way a country’s electoral system is structured usually determines how many competitive parties that country will have.
Here’s how it works. First, when each district gets only one legislative seat (known as a single-member district, which we have in the United States) and, second, when the election’s winner takes that seat, then the system tends to have two dominant parties.
In such a system, all a party needs to win is more votes than the other side. That winner-takes-all nature of single-member districts encourages broad coalitions to form before elections. https://www.washingtonpost.com/news/monkey-cage/wp/2016/08/02/could-a-third-party-candidate-win-the-u-s-presidency-very-unlikely/
It's worth reading the entire article but the takaway seems to be that in order for a third party to win the two party candidates have to be caught on video kicking kittens or a third party has to play the long game by working districts and establishing coalitions.