My five key factors:
1) Trump's success with evangelicals. I did not see that coming and really the nomination was over at that point because that meant Trump would be huge in south which, when combined with northeast, gave him a big headwind.
2) Kasich utter flop in Midwestern states. If you can't win Mich, PA, and IN and you're from Ohio, where can you win?
3) Failure of Florida politicians to win Florida. Marco's & Jeb's faces go on Wall of Shame.
4) Ego-driven delusional Republican politicians stayed in far longer than they should have. Jeb stayed too long, Marco too long, Kasich too long. Ben Carson too long. Only modest ones who got out in timely fashion were Carly and Walker.
5) Propaganda organ Drudge Report.