Here's one view that I think resonates with a lot of Trump supporters, for the sake of trying to understand what is going on:

There is a very large portion of the country for which the economic numbers you're seeing are not translating into a better quality of life. The disconnect between main street and Wall Street is hitting hardest in the very same places where Trump is winning the most, and that's not a coincidence. Huge swaths of America are stagnating and being left behind while Wall Street, the media, large corporations and establishment politicians pat each others backs and line each others pockets and leave the rest of the people out to dry.

Almost all the Trump talking points that have been derided in various ways by Trump's opponents (not entirely without justification, either) have common economic threads in them - "bringing back" jobs, reducing illegal immigrant competition for jobs, reducing competition from foreign countries, etc. They're all things that appeal to people that want a good job and a better life but can't find them. And those people are voting.

I think Bernie Sanders would have stood a very good chance of beating Trump, because he appealed to those same sentiments. Hillary Clinton does not, despite beginning to pick up Bernie's talking points later in the race. She's the quintessential entrenched establishment candidate, and represents that same status quo that economically disenfranchised people blame for their pain.