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?
Also, not all races are contested by both Democrats and Republicans. Sometimes the third party is the only one running for a position, sometimes it’s just one third party vs the Republican.
Local races are significantly different.