Bernie Sanders is a bad loser

From Jonathan Chait:

...The system isn’t rigged. Clinton is going to win the nomination because she has won far more votes. She currently leads with 55 percent of the total vote to 43 percent. That’s fairly close for a primary, but it’s not Bush-versus-Gore close. It’s not even Bush-versus-Dukakis close (the 1988 election, widely seen as a landslide, was settled by less than 8 percent). 

Clinton’s lead in pledged delegates is proportionally smaller than her lead in total votes because Sanders has benefited from low-turnout caucuses. 

Yet Sanders has enjoyed astonishing success at framing his narrative of the primary as a contest that, in some form or fashion, has been stolen from its rightful winner. His version of events has bled into the popular culture and fueled disillusionment among his supporters...

See also this.