If people choose to not vote because they don't feel represented, that's a valid stance. Voting is not a game of obedience, and it's up to the parties and the candidates to decide on a nominee that captures the interests and passions of voters.
Hillary is does not meet the ideals of many progressives, so they don't want to vote for her. You don't get to say to them that they should. They get to say to Hillary that they don't want to. That's how it works.
Now, I personally am a socialist compared to Hillary and am way, way to her left, and I can justify voting for her based on a "lesser of two evils" argument. In many other aspects of life I have to suck it up and compromise, so I can stomach doing it on the ballot easily. I will be voting for Clinton in the general election.
But I absolutely do not blame progressives for not wanting to, and nobody owes Hillary a damn thing.