Why were the national polls wrong?

[barfbag]

Maybe I'm having amnesia from six months ago, but what is the currently accepted explanation for why Obama overperformed all the national polls?

4 million conservatives never showed up to vote. They may have overestimated their chances or decided that a non-Christian candidate wasn't worth their time. Polls always show Democrats winning which is why I was so stunned on election night after seeing Romney winning for the weeks leading up to the election.