That, and she's not running.
Hello? She has a campaign going, of course she is running. Unless you meant Warren.
Obama veeeeeeeery narrowly beat her by turning the Iraq war into the issue that set them apart. Iraq was kinda a big deal in 2008. I dont believe there is a similarly sized issue for Sanders or Warren to exploit despite the cute attempts by the republicans to make it look like there is. And while Obama wasn't the highest level politician, I for one first heard his name 4 years earlier. The closest comparison I can see today is Elizabeth Warren but she is no Barrack Obama and wall street reform is no Iraq war sized political issue.
She has other chinks in her armor, and I didn't watch the 2004 DNC keynote address, so he was unknown to me at least. I know he barely beat her, I remember that long drawn out Democratic primary, the point is that it would be difficult, but not outright impossible.
Also, Warren isn't running anyway.
We'll just have to wait until the Democratic debate to see how the others differentiate themselves from Clinton. Would have been nice to have more debates on the Democratic side, though the Republicans need those more badly than the Democrats do.
Edit: Also, don't underestimate Murphy's law (or maybe the political version of Chekovs gun) there is always the chance for something major to happen between now and the finalization of the Democratic nominee that a challenger can latch onto and use.
Edit2: And of course, there's that Trump monkey wrench ready to dive in and screw things up, he's like the wildcard of wildcards. He's more of a loose cannon to the Republicans atm, but at some point, those shots could hurt the Democrats, you just can't predict (to an extent) what he'll do.