Well, there are a number of reasons:
1) Social conservatives stayed home. Somebody like Huckabee might have brought more of them.
2) Anti-Mormon bias. Possibly, among Hispanics. Look at Miami-Dade turnout in Florida, which was only 67%. Low turnout in rural counties across the state compared to other counties.
3) Working-class whites did not like Romney. Bain, his richy-ness, outsourcing, etc. Ohio and Pennsylvania are good examples where white turnout was not at par.
4) Libertarian voters - why vote for RomneyCare?
Romney ran a better campaign than McCain, but not nearly as good as Obama.