I am aware of the first-past-the-post system, but UK and Canada also have the same but their third parties gain national/federal seats.
In the US, it’s always either Democrats or Republicans. There are third parties on state and county level but never succeed on the national level. How come? Is the electoral college somehow impeding third parties?
The only real reason is lack of ranked choice voting. With the current system a choice is made that is either a win or a loss. This creates a binary system of Republicans or Democrats. Ranked choice allows for an automatic fall back. If you vote for a third-party candidate that you really like under ranked choice, this is not a protest vote. If your candidate doesn’t win you provided a fall back to a larger party candidate.