That's true - there is no way to tell for sure what the racial, religious, or other demographic attributes of those missing "potential votes" were.

Lots of people are completely disgusted with both Obama and Romney (for a variety of reasons) and decided not to vote in protest. I'm one of those who decided to go 3rd-party in protest instead of not voting at all, in hopes that it might help get them up to the 5% mark. Oh well.

As far as those in the "missing" group who have traditionally voted Republican goes, three reasons rise to the top of my mind as to reasons why they probably sat home and did not vote for Romney:

a) The Mormon issue bothered them too much.

b) The over the top amount of flip-flopping and blatant lying was just too much to condone.

c) They finally woke up and realized that they are in in the class of folks (working poor & middle class) that Romney's class (super-rich) will continue to use and then piss on, and they finally stopped voting for those who do not have their best interests in mind. That 47% video really drove that home for a lot of people.