The best thing I've read that explains the rise of Donald Trump—a cottage industry now—comes from George Packer in The New Yorker, who doesn't pull punches (calling Trump, among other things, “a celebrity proto-fascist with no impulse control”) but is measured while discussing how we got here; how Trump is both the same and different:

Republican Presidential candidates received majorities of the white vote in every election after 1964. In 2012, Barack Obama won about forty per cent of it, average for Democrats in the past half century. But no Republican candidate—not even Richard Nixon or Ronald Reagan—made as specific an appeal to the economic anxieties and social resentments of white Americans as Trump has. When he vows to “make America great again,” he is talking about and to white America, especially the less well off.

More, Packer doesn't dismiss these Trump supporters:

White male privilege remains alive in America, but the phrase would seem odd, if not infuriating, to a sixty-year-old man working as a Walmart greeter in southern Ohio. The growing strain of identity politics on the left is pushing working-class whites, chastised for various types of bigotry (and sometimes justifiably), all the more decisively toward Trump.

This is probably the key line: 

Trump has seized the Republican nomination by finding scapegoats for the economic hardships and disintegrating lives of working-class whites, while giving these voters a reassuring but false promise of their restoration to the center of American life.

I keep going back to these four words:

Social progress
Economic regress

During my lifetime (b. 1963), we've had social progress (Civil Rights Act, Voting Rights, Title IX, Lawrence, Obergefell) but economic regress (rich richer, etc.). More groups of people now have the chance to be part of the dwindling middle class.

Worse, GOP candidates have appealed to those suffering from economic regress by blaming social progress. It's mostly veiled—Goldwater's states' rights, Nixon's law and order, Reagan welfare queen, H.W.'s prison turnstiles—but it's there. And once in power, their policies wind up increasing economic regress while continuing to blame social progress.

Trump, as Packer says, is simply more blatant in his scapegoating. It's a formula will keep working until Americans wise up.