The nature of FPTP elections makes every election, functionally, the lesser of two evils. Anyone voter who isn't civically illiterate understands this, and votes for whichever of the two main parties presents a less fundamental affront to said voter's sensibilities. For actually-not-that-complicated game theory reasons, the only rational justification for voting third party is if both main parties are exactly equivalently evil, from the voter's perspective. This is only really possible if the voter doesn't really think about the parties, their platforms, or their histories much at all.