[chongli]

It's due to the voting system. Plurality voting systems tend towards two-party rule. This observation is known as Duverger's law [1].

Now, if the US switched to a radically different voting system today, we probably wouldn't see a flourishing of outside parties tomorrow; it could take a little while. But people sympathetic to outside parties such as the Greens or the Libertarians would be far less reluctant to vote for them if it didn't mean splitting their vote and causing the "lesser of two evils" to lose their local district.

[1] https://en.wikipedia.org/wiki/Duverger's_law

[ethbro]

Thankfully, that's gradually changing.

This November, Maine will officially be using ranked choice voting for US House and Senate seats.