Third-party candidates have difficulty winning elections in the United states because

nearly all elected officials are selected by

Third parties face many obstacles in the United States. In all states, the Democratic and Republican candidates automatically get on the ballot, whereas third-party candidates usually have to get thousands of signatures on petitions just to be listed on the ballot. The state and federal governments, which make rules governing elections, are composed of elected Democratic and Republican officials, who have a strong incentive to protect the existing duopoly. Also, third-party candidates often face financial difficulties because a party must have received at least 5 percent of the vote in the previous election in order to qualify for federal funds.

Third-party candidates have difficulty winning elections in the United states because

nearly all elected officials are selected by "Single-member districts".

Explanation:

Obstacles for the Third-party candidates to win an election are

Election ballot: To get on the ballot the third party must need a huge number of people's signature on the petition. Single-member districts: In our government no matter how close the election is only one candidate won. Financing: It cost millions of dollars to run a presidential campaign and mostly very few support the third party.