It’s a first past the post system with only two major political parties. That means the choice is either the selected Democrat or the selected Republican, who are elected via a complex primary process that differs from state to state.
Voting third party in the U.S. achieves absolutely nothing. Especially when there are almost never third party choices for lower office, aside from the libertarians and they’re nuts. If you are determined to not vote for any Democrats or Republicans, your vote has the same effect as staying home and not voting.
I would love this to change, but I don’t foresee that happening anytime soon.