He got elected for two main reasons, in my (humble) opinion:
He was an “outsider”, and Americans don’t really like or trust ‘career politicians’. They especially didn’t like Hilary Clinton (for several reasons). So voting for him was “sticking their finger in the eye” of the political establishment.
The Republicans have gamed the voting districts in several states (N.Carolina, & Wisconsin, just to name two), using “gerrymandering” techniques to redraw voting districts in their favor. They’d also purged voter lists of people who were likely to vote Dem. Which they’d been doing since the 2000 elections.
The R’s also have better skill with manipulating voters using ‘emotional’ issues using the media. Also, he’s obviously racist & racist Americans who didn’t like Obama voted for him.