No non-incumbent candidate since World War II has come into a primary race with bigger advantages than Clinton did. Probably more famous than any politician aside from President Obama himself, she had the entire party infrastructure so firmly wrapped up that Sanders barely even tried to get any endorsements. He couldn’t even secure the governor of his own state, where he’s been a dominant political force for decades.
Sanders is 74 years old. He’s an unabashed socialist in a notoriously red-phobic Democratic Party — which he did not even join until this campaign. He’s from Vermont, the second-smallest state in the nation, and as a result was barely known at all outside New England. The state is also extremely white, and so Sanders had almost no base among minority groups, particularly African-Americans, perhaps the core constituency of the party. He’s not a particularly great orator; his speeches tend towards the laundry list, not the soaring rhetoric of Obama.
Furthermore, Sanders is a white heterosexual man in a party that has long been anxious for more diverse representation, in keeping with its electoral base. Just look at how Martin O’Malley, who almost certainly would have been a contender a couple cycles ago, has utterly failed to gain serious traction. In 2008, Obama gained significant support for being potentially the first black president, and rightly so (though it was his record of being against the Iraq War in 2002 that put him over the edge). In 2016, Clinton would be the first woman president, and can thus claim representational value as being the first female president — again, rightly so.