It is clear that Bernie was hoping to replicate Trump's path of winning the nomination by attacking the party as an "outsider." There will always be a percentage of folks who will be drawn to an insurgent as a way of giving the system the bird. Look at your down ballot races when an unknown will still manage to get about 30 percent of the vote against a well established incumbent.
Trump captured the Republican nomination by catering to that 30 percent. Bernie was hoping to capture that same magic by trashing the Democratic party. Yet, unlike Republicans, Democratic leaders put the American people above their personal political ambitions. Of course, Bernie thinks that is not fair.
https://www.nytimes.com/interactive/2016/04/24/us/elections/trump-gop-delegate-system-rigged.html
Donald J. Trump has been a vocal critic of the Republican Party’s nomination system, calling it “rigged” and accusing other candidates of stealing his delegates.
Mr. Trump has faced several setbacks during the delegate selection process, a complex system of rules for choosing national convention representatives that was put into place long before Mr. Trump became the frontrunner.
Not all delegates who have been selected to represent Mr. Trump are fans of his. The majority of delegates are being selected through a series of elections at local and state conventions. In some states, some of Ted Cruz’s supporters have been selected to represent Mr. Trump at the convention. Mr. Trump has called them “double agents.”
This may not matter at first because they will be bound to Mr. Trump during the first nominating ballot. But if no candidate reaches 1,237 delegates and there is a second ballot, these delegates will be free to vote as they please.