I don't necessarily believe Bernie won't stump for Hillary after he drops out, but the longer this race goes on, the harder I have to imagine it'll be that Bernie's supporters will actually bite and flock to Hillary's camp in November.
When you've been telling your supporters -- many of them first-time voters whose first experience of politics is this election cycle -- for the better part of a year that your primary opponent is bought and paid for by Wall Street, and that she's the face of a Washington/corporate/special interests establishment that has sold your jobs to other countries and rigged the economy against you, how in the hell do you turn around and then convince them, "well, but actually, you should vote for her after all?"
Bernie himself might acknowledge the importance of getting a Dem into the White House, but how do you tell millions of people who were inspired by a political revolutionary that they now have to settle for (in their view) the lesser of two evils? Especially when the other nominee has, like Bernie, been long flaunting his anti-establishment and special interest-free credentials?