They'd rather lose with Hillary than win with Bernie.
Tell you what jfern...prove that Bernie is guaranteed to win and that Hillary is guaranteed to lose and I'll switch to supporting him. 
Of course there are no guarantees, but all the evidence has Bernie doing much better.
What evidence? There is no evidence. Pointing to a poll is not evidence, especially when Bernie has not gone through media vetting or the GOP attacks that will surely come his way. How does Bernie defend himself against an ad that says "Bernie Sanders, a socialist, want to take away your private health insurance and raise taxes on all Americans," juxtaposed with videos of Bernie says these very things.
Fact is we don't know how Hillary Clinton or Bernie Sanders will hold up in the general. We can't predict the future. If polls were indicative of who will be President then in 2012 Romney should of been elected, in 2008 McCain should of been elected, in 2004, Dean should of been elected, in 2000 Gore should of been elected, in 1992, Perot should of been elected, in 1988 Hart should of been elected etc etc etc.
The Washington Post has been doing those attacks against Bernie for months now. Meanwhile, how many people know about the Clinton Foundation pay to play? Or that they take money through a Canadian foundation if they really don't want you to know who they took money from.
Obama usually led Romney by 1-5 points for the 2012 campaign. It was Obama +2 on May 13. The polls didn't really vary that much. So if this is like 2012, Bernie winning would be a sure thing.
As for 2008, what changed was the worst recession in 80 years helping Obama. Hillary has more to lose there if there's a recession before the election since she's more closely tied to Obama and the status quo.
Poll
