Some good discussion here about how these people fell into it, and what to do about it.
I think it's a confluence of Trump appealing to large groups of previously apolitical people, right-wing media's increasing demonization of the left for the last 20? years, and social media algorithms.
You get large groups of people suddenly paying attention because they like the orange man, what news they have received or paid attention to the last X years has constantly told them the other side is basically evil, then some whackos with youtube channels peddling conspiracy theories start getting pushed at them in a feedback loop. Oh, and let's not forget Russia's social media manipulation to help kick things off.
The good news, perhaps, is that the social media part can be fixed if Google/Facebook are willing to do so. And without the orange man hopefully people start to lose interest in politics again.