"Make America Great Again" was less about "Great" and much more about "Again"
Trump is simply not difficult to understand for most people. Almost everybody knows somebody who has some of his characteristics, both the good and the bad. He is bigoted while genuinely thinking that he isn't. He is believable in his denials because he is Archie Bunker with a private plane. He may be disagreeable but he is understandable.
There is nothing we don't understand about him - eating KFC on your plane is something everyone can understand. The KFC part of it, the plane part becomes secondary, amazing as that may seem.
His vanity, his thin skin when it comes to criticism, his desire to be considered smart, his cheating at golf, his desire to change the wife for the newer model, all of those are understandable. No mystery about them for anyone. We may chose not to live that way ourselves but we do it as mastery of self-control over our more base impulses. He is developmentally challenged in self-control. And like with a developmentally challenged person, many seem to be willing to be tolerant of his glaring defects.
In particular when that is contrasted against something very complex, such as the Clinton monolith where we feel that we never really get 100% of what is behind it all. The Clintons are a bit too much like the Underwoods in House of Cards. Impressive and capable but there is always that nagging question of just how trustworthy.
With Trump we don't have to believe in any grand vision or a complex plan. To him everything is simple the same way that to a man with a hammer everything looks like a nail. He is simple so he sees simple everywhere. And that just appeals to a lot of people whether or not it is an accurate understanding of the world.
He resonated with the "inner redneck" in people, with the inner primal male who would ogle the neighbor's wife if only for fear of getting caught, he even resonated with women precisely because he is simple. As a woman, you could be attracted to slick Willy but never quite know what he is up to. With Trump you pretty much know what you are dealing with in 15 seconds or less.
So all the irritation of being told by the cosmopolitan elites that we don't know any better; the frustrations of being proven dumb by every next generation of iPhones; the feeling that somebody is just not telling the truth when they are telling us that in 50 years the seas will rise; the anger at the "swamp" in Washington where everyone seems complicit in an elaborate dance to keep things rotating nicely between the two parties. All of that culminated in just enough people being willing to say "to heck with it, how much worse could he be" and upset the apple cart.