Commenter Brian makes an observation "No one is talking about how the polls actually nailed Obama's number. Obama didn't lose this election. He stayed steady and Hillary surged ahead." That seems to be true. Here's a chart comparing the actual results to the most recent Pollster.com current standard estimate polling average.
Just as Brian says, the difference between the Obama poll level and the Obama vote total level seems to just be your basic statistical variance. The pollsters underestimated Clinton's level of support. People who were undecided as of the last round of polling seem to have gone overwhelmingly in her direction.
[also note the relevance of this to Wilder/Bradley effect speculations]