This was supposed to be an election in which the voters were really interested. That turned out to not be true. As the top chart (from the Washington Post) shows, a smaller percentage of registered voters participated in the 2016 election than in any of the three elections that preceded it (2004, 2008, 2012). Only 56% voted, while 58% had voted in 2012, 61.6% in 2008, and 60.1% in 2004. You have to go all the way back to the 2000 election to find a smaller percentage voting (54.2%).
Donald Trump had bragged that he would increase turnout for the Republicans. That did not happen. Only 59.7 million Republicans voted -- 1.2 million less than in 2012, 0.3 million less than in 2008, and 2.3 million less than in 2004. The GOP turnout was down slightly from previous election.
If the GOP turnout was down, then how did Hillary Clinton lose to Donald Trump? The answer is that the Democratic turnout was down even more than the GOP turnout. About 6 million fewer Democrats voted than voted for Obama in 2012. And 9.8 fewer Democrats voted than did in 2008.
How did this happen? We know that Democrats didn't want Trump to become president. Were they lulled into thinking that Clinton had the election in the bag, and didn't think their vote was needed? Did too many Bernie supporters stay at home -- still mad that their candidate didn't get the nomination? Did many Democrats get fooled by the right-wing (and left-wing) lies about Clinton? Was it a combination of all three?
Whatever the reason, Democrats have only themselves to blame for the Trump presidency. Their mush vaunted "ground game" simply failed to turn out enough votes. They would have won if they'd just come close to the votes they turned out for Obama. But they didn't.