I think the problem is that he's simply the wrong messenger to run against a political machine. He's not a good politician, doesn't really appeal to a broad coalition, and is 70+ years old. He's Ron Paul for democrats.
It's hard to appeal to low information voters when you don't really do anything to stand out. While running against a candidate with full name ID who is basically running as the heir of a popular president. Those voters are going to blindly vote for Hillary because she's next up, has establishment credibility, etc; not to mention the historical nature of her campaign.
To use GAF as an example, how many times have you noticed drive-by posts about Sanders allegedly having nothing to say about civil rights, no record on civil rights/criminal justice, "marching with King doesn't matter," etc. And when you post your information that contradicts those talking points what happens? Some admit they had no idea, others say nothing, but the status quo remains in place. I don't believe Sanders can combat that, given his limitations as a politician. If he was 20 years younger and had the charisma of a Barack Obama (or even a Howard Dean) sure, I could at least buy the idea he could give Hillary some trouble in certain states.
But that's not the case. I don't see him winning 5 states against Hillary. But I'd hope that his supporters realize this is a very important election, and sitting out because your guy lost would be pure stupidity.
