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?
Ross Perot had a shot in 1992 but blew it by promoting conspiracy theories (that may have even been true, who knows) about Republican infiltration at (iirc) his daughters wedding.
And the split in the vote let Bill Clinton beat the incumbent Bush.
The only way a third party takes a seat is when they capture the attention.
Locally, that’s easier. Less people to win over, you get to focus on local accomplishments, etc. But national you need to become the national conversation or too many voters wouldn’t know who you are.
Ross Perot never had a real shot. The ruling parties would not have permitted it.
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.