Actually, it is a bit of a surprise (well, not really):
The thing about Bernie is that he could actually win the election: his polling numbers are much higher than Hillary's when you poll everyone, not just Dem voters. He was a lot like Obama in 2008: he energized the youth vote (youth apathy has long been a big problem for the Dem party), and if he was on the ticket they'd be out there in droves voting for him, and also for down-ticket Dem candidates.
This just isn't the case with Hillary: the younger people either despise her or at least have no enthusiasm whatsoever for her.
She's still likely to win (but it's not a sure thing), but only because the Rep candidate is so awful. If she were running against McCain or Romney or even Jeb, I'd fully expect her to lose.
So the only thing that's surprising is that the DNC is so dumb that they'd double down on such a lousy candidate, banking on the Republican candidate being even more horrible (and they decided to back her *long* before anyone had a clue that Trump was going to win; most people probably assumed that Jeb would).