Looks like I am not the only one asking this question. Roger Cohen also asking the question today. I think the first response is most critical to me.
How is this possible? It is possible because spectacle and politics have merged and people no longer know fact from fiction or care about the distinction. It is possible because fear has entered people’s lives and that fear is easily manipulated. It is possible because technology has created anxiety-multipliers such as have never been known before. It is possible because America is a country living with the dim dissatisfaction of two wars without victory and the untold trillions spent on them. It is possible because a very large number of people want to give the finger to the elites who brought the crash of 2008 and rigged the global system and granted themselves impunity. It is possible because of growing inequality and existential dread, especially among the white losers from globalization who know minorities will be the majority in the United States by midcentury. It is possible because both major parties have abandoned the working class. It is possible because a lot of Americans feel the incumbent in the White House has undersold the United States, diminished its distinctive and exceptional nature, talked down its power, and so diluted its greatness and abdicated its responsibility for the well-being of the free world. It is possible because the identity politics embraced by urban, cosmopolitan liberals have provoked an inevitable backlash among those who think white lives matter, too. It is possible because Trump speaks to the basest but also some of the most ineradicable traits of human beings — their capacity for mob anger, their racist resentments, their cruelty, their lust, their search for scapegoats, their insecurities — and promises a miraculous makeover. It is possible because the Clinton family has been in the White House and cozy with the rich and close to the summit of a discredited political establishment for a quarter-century now and, to people who want change or bridle at dynastic privilege, that makes Hillary Clinton an unattractive candidate. It is possible because history demonstrates there is no limit to human folly or the dimensions of the disasters humanity can bring on itself.