I suspect greed and vanity may have been a factor. Trump didn't campaign in CA, he chose to spend his time in places where he could win.
Hillary DID visit CA, several times. Why, when you're ahead by double digits?
Money and fame. Big Dem donors, glitzy parties, celebrities groveling at your feet. While she partied, Trump stole the rust belt.
I remember Soros saying Hillary would win but that Trump might win the popular vote.