That's simply enough to answer...
1. Anti-establishment.message.
2. Pro-American worker message.
3. Drain the swamp message.
The "hope and change" message dominated in it's day too.
I think it's exactly this.
I also think there's much to be learned there from a business sense on communication.
I think one of the more interesting things about Trump is his communication. Where's it's mocked by most, the reality is I think he stood out in the primary field for one reason - he was clear.
I've heard Donald Miller talk about this some. He talks about Jeb Bush and how Bush was imminently qualified to be President. And very intelligent. Has written books on policy and such.
Miller said he'll go in front of large audiences and ask, "What was Jeb Bush's vision for America?" And he'll get crickets.
Even asking "What was Hillary Clinton's vision for America?". He said sometimes you'll get a few hands with something about "I'm with her".
When he asks "What was Donald Trump's vision for America?", regardless of how anyone voted, you get "Make America Great Again".
It's clarity.
People rarely move towards unclarity. They move toward where they get clarity. And he was clear.
Now of course, whether the subject he's clear about is the right answer or not is an entirely different and more important question. But you have to get the first one handled first. And Trump seems pretty good at the clarity thing.