Nate Silver, who has been as wrong about Trump as, well, everybody, explains why he thinks he was so wrong, in a single tweet:

Updated post-Trump priors:
1. Voters are more tribal than I thought.
2. GOP is weaker than I thought.
3. Media is worse than I thought.

— Nate Silver (@NateSilver538) May 1, 2016

Let me take each in reverse order.

Media is worse than I thought.  By this, Silver is saying that the media bias for covering Trump, and not holding him to the fire in terms of his vetting his claims, is unprecedented.  Trump has exploited these weaknesses.  Read more here.

GOP is weaker than I thought.  Silver here is referring to GOP, the party, not GOP, the voters.  “The elites” might be a way to put it.  Usually, the party establishment has some control, and while the voters have influence over who IS the “establishment”, they are mostly along for the ride on any given election.  That paradigm fell apart for the GOP this cycle.  Trump is the candidate who finally figured out how to exploit the fact that much of the Republican voter base does not share the policy preferences of the Republican donor class, and that it is therefore possible to win the nomination without being saddled with their unpopular policy preferences.  Read more here.

Voters are more tribal than I thought.  This is the most intriguing for me.  We may have entered a world where the left-right paradigm no longer exists.  Silver explains it this way:

But whereas Cruz offered a mix of anti-establishment-ism and movement conservatism — and whereas Marco Rubio offered movement conservatism plus a strong claim to electability — Trump’s main differentiator was doubling down on cultural grievance: grievances against immigrants, against Muslims, against political correctness, against the media, and sometimes against black people and women. And the strategy worked. It’s a point in favor of those who see politics as being governed by cultural identity — a matter of seeking out one’s “tribe” and fitting in with it — as opposed to carefully calibrating one’s position on a left-right spectrum.

There is a tribe of aggrieved electorate — not necessarily conservative but (I would argue) weened on conservative talk radio and media.  That’s the tribe that Trump has tapped into, and it is a large one.  Large enough, it seems, for him to win the nomination.

Should we stop looking at the electorate on a left-right spectrum, and start thinking of it as “tribes” to appeal to?  Or is the Trump phenomenon a one-off thing?  Time will tell.