Then you run into the systemic failures of the current political and electoral systems in the US (and some other countries with electoral systems dating back to a similar period) - that voting for a third-party candidate is to throw away your vote for the most acceptable of the two "provided" candidates, and practically guarantee that the least acceptable wins due to the divide in votes.

There are, of course, a variety of electoral systems that allow and encourage third-party candidates to have a chance, but it's very, very hard to change an electoral system that got everyone in the political system where they are, to one which provides them new competition. See parallels with regulatory capture.