Working class white independents stayed home. They didn’t like anyone running. Some areas had a very large dropoff for both Romney (from McCain of all people) and Obama from 2008. That was even in areas McCain lost and Romney won.
It’s even odder than it seems.
In places like Oklahoma and Ohio, there was a dropoff in votes for Romney compared to McCain. But Romney got a million more votes than McCain.
I haven’t run the numbers so I don’t know which states gave the boost to Romney over McCain.