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.

In addition to what the other comment said, FPTP causes the Overton Window to continually shift right. As the political right party takes even more radical positions and rallies/propagandises people to support that radical position, in order to gather enough voters to stand a chance the political left party moves rightward to pull the more moderate right-wing voter's.