1. Sanders is largely unknown. What little is known is that he thinks the system is rigged and he’s standing up for the little guy. He wants to raise taxes on someone else and give more to everyone else. As we've seen with Donald Trump, populism is a very strong message right now. People really don't know more about his plans and programs and other ideology on foreign policy, but what little they is pretty attractive. He's selling a lot of what Trump’s selling, but without being offensive, demeaning, and nasty. Sanders seems like a nice genuine guy. Sanders has also benefitted from Hillary Clinton not going nearly as negative on him as the Republicans have gone on each other. The problem Sanders would have in a general election is that Republicans would go negative and America would learn a lot more about him. Some of the attacks would be misleading while others informative. Regardless, his popularity would decline.
2. Clinton is known and people don’t like her. Thus, some independents and even Republicans who don’t like Trump, Cruz, or even Kasich won’t vote Clinton. Clinton has the double edged sword of those that like her really like her and will stick with her, but those that don’t would never vote for her. Were Sanders competing against else he wouldn’t do as well. It's not like any other opponent would be under the threat of indictment.
3. Some of Sanders’ support comes from far left independents who’d probably sit out or vote Green rather than vote for Clinton. I think this is the smallest group of the three, as most on the far left would vote Clinton rather than see a Republican win.
By summer it won't matter that Sanders polled better. He's not getting the nomination, so we'll never know how his candidacy could've played out.