BALTIMORE, MD - APRIL 23: Democratic Presidential candidate Senator Bernie Sanders speaks to a crowd of thousands who attended his A Future to Believe In Baltimore Rally at the Royal Farm Arena in Baltimore, Maryland on April 23, 2016. (Photo by Marvin Joseph/The Washington Post via Getty Images)

There are at least two reasons, one of which is far, far simpler than you might expect:

A great many Sanders supporters are only involved because of him; no one else can get them to the polls.

Neither Clinton's campaign, nor the media, nor the DNC really seem to understand this, or take it seriously. People love Bernie because he's so different; he, and only he, has inspired them. It's not a matter of people deciding en masse not to vote for Clinton; it's that a good many of Sanders' supporters would only ever vote for him.

Context

Most people hate politics; they think it's a sham, and they don't trust politicians. There are many explanations as to why Sanders has been able to reach them, but it all boils down to this: to them, Sanders is a profoundly different kind of candidate. It's not necessarily about Clinton; she just represents and employs the same old politics-as-usual that they can't stand.

Contrast

Sanders' supporters view his campaign as having been honest and issue-oriented. Clinton's tactics, while they may seem underhanded, are not so unusual; but when used against someone like Sanders -- who has, in his supporters' view, steadfastly refused to go there himself -- they seem particularly odious. Needless to say, that makes Clinton's prospects of wooing Sanders' voters even dimmer.

The Clinton camp has been quick to complain about Sanders' language or tone; ironically, they seem not to have considered how their own approach impacts the voters she might need to win over someday.

Controversies

Add to this the perception that the DNC have done their best to rig the primary in Clinton's favor; the fact that voting irregularities have all seemed to happen in states that Clinton has won; and that the media have not covered Sanders properly; and you could develop a powerful impression of unfairness.

The Ultimatum Game

There is an experiment called the ultimatum game. It shows that people will, time and again, turn down an offer that would benefit them, simply because they feel the person making the offer would benefit more, unfairly. My father once called it "biting off your nose to spite your face"; but I think it might better be characterized as not giving the satisfaction. It is totally irrational; and it is entirely predictable.

... Que la raison ne connait point

That is where many Sanders supporters may find themselves if Clinton wins the nomination: feeling that she will have done so so unfairly, that they will simply not be able to bring themselves to vote for her. It is, given the likely Republican alternatives, totally irrational; but it is, also, entirely predictable.

It's all Greek tragedy to me

And so we have the other reason, and the dark truth of the Democratic primary: Clinton's campaign has included so much that Sanders supporters loathe about politics, that she may have alienated them beyond recovery, and created an enthusiasm gap that is too broad to bridge. If she wins the nomination, her chosen path -- to run roughshod over Bernie Sanders -- could ensure her eventual defeat.

In that case, Clinton and the establishment she represents will have no one to blame but themselves.