It’s not terribly difficult to get on the ballot. But each state has their own requirement so the third party candidates you see may not be on every ballot nation wide.

It’s not just the media that promotes the two candidates. It’s the billionaires and corporations who invest the most money in promoting someone they find favorable to their interests. Given the bankroll surrounding these two parties, the media really don’t have much choice but to reflect their advertising efforts.

In the US, we have what’s called first past the post elections. That essentially means the first person the get the most votes wins. In this case, it’s electoral votes, not individual votes.

Had the US had something like ranked choice voting or star voting in general elections, third party candidates would be given much more attention.

Given the voting system we have, we mostly vote in a manner that prevents the election of the most popular candidate we don’t like.