In addition to @Wanderfound’s point about disenfranchisement, don’t underestimate the media’s ability to persuade people to vote against their own interests:
I bet you’d be hard pushed to find a single lib dem farmer in the UK but given that the EU Common Agricultural Policy accounts for 55% of average farm income and a no deal brexit will mean a 60% tariff on all meat exports to the EU, the lib dems were the only party in the last election that would have even come close to protecting their interests.
Instead, the Daily Mail et al use rural voters’ innate fear of the unknown (how many sheep farmers in Norfolk have even met a Muslim or a black person, let alone built up some kind of empathy through friendship?) to push through the deregulate-and-privatise agenda of a handful of billionaire media barons.
In the American example, you have a focussed right-wing media that taps into resentment of decades of capital flight and brain drain to the coasts and tells you that Obama and Clinton represent “cosmopolitan liberal elites” who don’t understand you. You look at the money that goes into stations like NBC and CNN, their focus on business news, their accents and their clothes, and you decide that you have nothing in common with them. Then you get a Fox or a Limbaugh or a Jones, who target their “news” at you directly, and you start to feel that these people are on your side.
It doesn’t matter that a functional minimum wage would help you directly, or that socialised healthcare would mean you could avoid bankruptcy in the event you get sick; how could you possibly vote for “crooks” like Obama or Clinton when all of the media you identify with is telling you that only the GOP is really looking out for you? That only Trump will bring back jobs for Real Americans Like You?
To be fair to them, it’s not like the DNC really represents their interests either…