People get more pragmatic the closer they get to an actual vote.
That comes from Joe Trippi, who managed Howard Dean's 2004 presidential campaign. In an interview with the Washington Post's Chris Cillizza, Trippi lays out a few reasons he thinks the road forward for Sanders is even harder than it was for Dean, including the fact that Hillary Clinton has the Democratic establishment locked down in a way none of Dean's opponents ever did.
But it's really that point about pragmatism that's most important. As Trippi says, "We drew large crowds and lots of energy but there was always a fear among the party establishment that we couldn’t beat Bush. In the end those kinds of doubts always move voters back towards the establishment’s candidate."
This is a dynamic that early polls miss. At some point, Democrats are going to watch Republicans coalescing around a candidate who can actually win — a Jeb Bush or a Marco Rubio or a Scott Walker. And they're going to imagine that Republican nominating the next Supreme Court justice, or deciding whether to go to war with Iran. And they're going to get scared. And then they're going to look for the Democrat who they think can win.
The challenge for Bernie Sanders — and it's a big challenge — is he needs to convince Democrats that the country that elected George W. Bush will elect a Vermont socialist. He needs to convince them that when the Koch brothers and Sheldon Adelson begin dumping money into the election, he'll be able to withstand their assault, despite his decision to forgo any kind of Super PAC support. As of today, few Democrats believe he can win the Democratic nomination, much less the general election.
Democrats aren't particularly inspired by Hillary Clinton. But they tend to trust that she can win a street fight with Republicans. To win, Sanders needs to convince them that they're wrong — about her electability as well as his.