In defense of the pollsters I'll say that their models aren't designed to account for the specific type of phenomenon that the Sanders campaign represents. Their models are based mostly around the depressing predictability of how voters react in the Skinner box environment of large, extremely well funded, elections.
It would be dishonest for pollsters to use a new methodology. Though I'll be interested to see if Nate Silver is open about this if asked. I understand his livelihood is based on the uncannily good models he uses, but he is a scientist and should be honest about the limitations of prediction.
There are some importantly unusual features to the Sanders campaign. The degree of how important they are, and the number of them, is what is going to show the limits to the utility of the models Silver uses.
Ask Silver if his models could have predicted the surge of Ross Perot in 92. And Perot was seriously lacking in several attributes that Sanders has in spades. Sanders has the record for championing people that the Perot campaign merely gave a nod to. Sanders has decades of experience with the legislative process. Sanders has decades of support for important civil rights causes under his belt. Sanders has decades of campaigning and politicking in unpleasant circumstances and environments behind him.
Though the main point of superiority is that Sanders is running on the most evolved version of the Democratic Party platform I've yet to see. Because Sanders has chosen not to take big donations from monied interests, he and his campaign could craft a platform that would have the utmost appeal to members of the Democratic party (and the nation as a whole).
Silver should consider the Perot phenomenon, look at how the Sanders campaign is greatly superior to it, and conclude he needs to come up with something more specific to this campaign. He might make just such an announcement if Sanders does a notch better than even the already revised upwards forecasts for the earliest contests and/or later very significant polling.
P.S. Only now did I really think about how Perot was up against a Clinton and a Bush. The huge difference her obviously is that Senator Sanders is competing in the Democratic primary, and not against Bush and Clinton at the same time. He doesn't get the spoiler vote though he does appeal to those who aren't happy with our system.