Bush got 62 Million votes and Romney got 57 million because Bush won and Romney lost. The 5 million votes you are talking about are moderates and independents who voted Democratic because Republicans chased them away
The effective distribution of independent and moderate votes was about the same. In fact, in the case of Al Gore, he actually got more of the independent and moderate votes. It wasn't bailing indies and mods who cost the GOP in '08 and '12, it was failure of the base to show up. This will continue to be a problem if the establishment moderates in the GOP don't wise up fast.
Bush didn't win because he was a moderate. He won because he was a social conservative and the Christian coalition is the largest single voting block in America. The GOP needs to realize the moderate/independents do not matter, they represent a tiny number and are not worth the effort to cater to. You're going to win a percentage of them and the opposition will win a percentage, and in the end, their votes will not determine the winner.
Basically, there are three groups the GOP should concern itself with... Moderate establishment types, Tea Party Conservative types, and Social/Religious Conservatives. If you appeal to two of the three, you will win elections. If you appeal to the later two, or all three, you will win elections in landslides.