The USA famously has a 2 party system. What you need to understand is that there is no law that says there can only be 2 main parties.
Also, the 2 main parties don’t conspire to keep things that way.
There are parts of the US Constitution that define how the President and Vice President are elected, which is using a “first past the post” method. And the various state and local elections also use a “first past the post” method.
Given that method of elections, a two-party system is almost guaranteed. Given three or more candidates, voters will usually develop a ranking of those candidates, even if only subconsciously. Something like, “I really want A to be elected. I could live with B. I really don’t want C.” But there is also the perception of who is likely to be elected. If B is more likely to win that A, many who prefer A will instead vote for B, because the possibility of C being elected is worse than accepting B.
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The two main party candidates will almost always be perceived to be the two most likely to be elected. That means that a lot of people who might vote for a third party candidate will instead vote for the more tolerable of the two main party candidates.
This makes it very rare that a third party candidate will win. And often when that happens, it means a change in which parties are considered the two “main” parties.
Essentially, the only way to have more parties is to change the way people are elected. And that will likely require changes to the US / state constitutions.
Edit: I originally type “first past the gate” instead of “first past the post”. I’ve corrected that.