Mitt Romney has been running for president on a continuous basis since some time in 2006. Try to imagine that you have been spending the last 6 years doing almost nothing but trying to be elected president. If it were me, some time during that 6 year period, other than in the last couple of weeks before the convention, I think I would have settled on a VP. Someone I knew, I trusted, with whom I was compatible, and someone I would be comfortable as president if I could not complete my term.
Is it possible that Mitt Romney really didn't have anyone in mind such that it was just a crap shoot at the end based on his reading of the tea leaves just prior to the convention? Sadly, this is probably true. Consider that he has been running for president for 6 years and seems not to have figured out what he would do as president.