As an outside observer to American Politics I don't understand why a Third party hasn't gotten more traction.
Both Dems and Reps have people in their parties who would fit into a third party but there has never been a truly successful one. Centrist party?
The biggest reason is because of the voting system within the United States. While many Western democracies use a proportional system of voting and vote by party, many English speaking countries and former British colonies, not just the United States, use a First Past the Post system of voting. In a proportional system of voting, seats in a legislature are typically rewarded based on the percentage of votes a party received in the election. But in a First Past the Post (we'll call it FPTP from now on) system of voting, voters typically vote for a candidate, not the party, and the candidate who receives the most votes wins.
Nations that use FPTP typically develop two-party systems ranging from the extreme where there are literally only two parties in power, like in the United States, or are de-facto two-party systems where you have only two parties that really matter and everyone else is pretty damn irrelevant like in the United Kingdom (Conservative/Labour), India (National Democratic Alliance/Indian National Congress), and Canada (Conservative/Liberal).
The main reason why FPTP systems typically evolve into two party systems is because unlike proportional systems where smaller parties can aim small and eventually grow from there by developing governing experience, smaller parties in FPTP systems don't get the chance to develop governing experience because they have no one in power that matters. Even though voters can be ignorant, they're not irrational. They're not going to give power to an idealistic party that has no governing experience like the Libertarians or Greens. Instead, they're going to give power to the experienced parties, the Republicans and Democrats.
Now the reason why the United States is more extreme than the other English speaking countries is because of its system of government. While the other English speaking countries use a form of parliamentary system of government, the United States is more like its brethren in the Western hemisphere, in using a Presidential system of government where the head of government is completely separate from the legislature. We elect our head of government as opposed to many other countries where the head of government is the leader of the party with the most seats.
While the other English speaking countries use FPTP, the parliamentary system they use at least allows for a few minor parties to get a few seats and be completely irrelevant. Sometimes, they may get lucky and be invited in a coalition government if no party barely gets enough seats. The combination of a Presidential system of government with a FPTP system of voting in the United States just makes things a lot more narrow and creates even less opportunity for minor parties to develop governing experience and power. There isn't much opportunity for a small party to shake hands and form a coalition with the Republicans or Democrats and a lot of it comes from the President being completely independent of Congress and when he appoints members of his cabinet outside of his own party, it's because of his own choosing, not because of any deals made with other parties.
And finally, just look at the utter size and scale of the United States, it's a huge country with very, very, very expensive media markets where only the parties in power have the ability to take full advantage of. If the United States were a much smaller country, like many European ones are, perhaps they would have better opportunities to market themselves. But the vast expanse of the United States that doesn't have a single city that is the center of everything like the United Kingdom, Japan, or France, can make it very difficult for smaller parties to develop the resources to make themselves known.