Hindsight

With the benefit of hindsight the 2016 US Presidential election turned out to be between the democrats, representing the stutus quo vadis, more of the same, the establishment; and Trump, a populist demagogue of the far right. So it was a battle between the far right and the center. The far right won because Trump said polpulist, radical things that made people vote for him because they were fed up with business as usual and wanted to vote for radical far right populism.

Bernie Sanders was about as far left as you can get in US politics and was also a populist in the strict sense of the word, funded as his campaign was by grass roots support. His views and policies are really only centre-left but would have been portrayed as radical left, and many of his supporters came out when he didn't win the nomination saying that, given the choice beween a vote for Clinton, a vote for Trump or not voting at all, they would not vote at all.

The electorate was in the mood to punish the establishment by electing a radical. They were given the choice between the establishment and the radical right. If Sanders had been allowed to win the nomination the choice would have been between a rightwing radical and a leftwing radical. A radical was going to win in any event and only the radical of the right was available as a choice. The result was really a forgone conclusion.

The DNC had polling to indicate  this state of the electorate but chose Clinton and therefore must shoulder some of the blame; but aside from that how can you call a system democratic  where

      "The party has won the popular vote in six of the last seven presidential
        elections",

but not won the presidency in five of them?