Voting third party in a first-past-the-post electoral system is a coordination problem. If I, as an individual, have some grab-bag of political stances, with varying weights of importance to me, it is likely that some issues will become a matter of "I am willing to cast my vote in any direction that minimizes the chances that someone with the opposing view does not take power". For example, for people who strongly oppose a backslide into Facism in the US, the only real choice in the last presidential election was voting Democrat, because while there might have been third party candidates that would have been better, the Dems were the most likely opposition to that to win. Absent extremely effective widespread coordination, a vote for a third party increases the probability of an unacceptable outcome.