Fiercely partisan voters on both sides of the aisle agree on one thing: If you vote for a third party or independent candidate instead of one with an "(R)" or "(D)" printed next to their name in the newspapers, you are basically just "throwing away" your vote.
In their political calculus, a principled vote is a wasted vote unless it's for a candidate that's polling as one of the top two in an election, and for them it's better to cast a damage control vote for the least bad option that has a chance of winning, than waste a vote on principle.