I’ve not scoured the other replies, but I don’t think it was much more than arrogance and complacency on the side of the Clinton campaign.
Hillary had most of the money, the media, and the corporate liberal end of the political class (probably most of it) on her side.
Her election to presidency was meant to be a formality, a coronation, a foregone conclusion.
The first arrogant tactic was the “Pied-Pier strategy”, thinking Trump would be so detrimental to the GOP cause that she’d use her media contacts to boost his exposure. Trump got billions of dollars worth of free airtime, which helped him beat his rivals.
The notion that Trump could win the nomination, let alone the Presidency, was openly laughed at.
Unfortunately for Clinton, most people aren’t rich liberals who could afford to care about the indulgent symbolism of a first female President, or that she was supposedly more qualified and experienced, or that Trump was openly derided by liberals as a clownish and utterly unqualified and incapable candidate.
Then you’ve the fallout of the 2008 crash, the relative disappointment and utter failure to change much of anything under Obama, and the perception of Clinton as a self-serving continuation of the status quo—there was little of anything to inspire anyone.
Clinton ran a campaign that utterly misjudged people’s mood. She represented a brand of politics that was increasingly seen as out of touch, elitist, and not in the people’s interest.
Her rhetoric was platitudinal and appealed to a supposed fear or how bad Trump would be—except no one who wasn’t already going to vote for Clinton cared.
Trump—despite also being wildly unpopular —made the best of the situation, and appealed to people’s sense of fear and frustration, which Clinton didn’t. It was just enough to get him over the line in enough of the relevant states.