Reagan almost unseated the sitting President for the nomination. Even at the convention it was unclear if anyone planned to bolt for Reagan and give him the nomination.
During that time, Reagan also wrote and spoke extensively to keep his name alive. As much as I hate to word, Reagan seems to have had a "destiny" that only needed to play itself out as his movement built. Huckabee and Romney seem more like accidents of the modern political climate, with no real staying power.
Yes, I know that Reagan was a "bigger figure" than Romney and Huckabee are now. The point is that every one of these cases is unique. In 1996, you could have argued that Dole would break the pattern of "runner up from last time around gets the nod" because he was so old. In 2008, you could have argued that McCain would break the pattern both because of his age and because of the party base being unsatisfied with him. But both times, the pattern held.
I don't see why being out of office for six years is a good enough reason to discount the "runner up from last time around gets the nod" this time around. Why would that be a big factor? It seems to me that, nowadays, the #1 way to get onto the fast track to top tier status in the primaries is being famous. Huckabee and Romney are now more famous than Sanford or Crist or Pawlenty or Thune. Unless one of those folks is tapped by McCain as VP, Huckabee and Romney will still be more famous than them a year from now and two years from now and three years from now, and will have an easier path to top tier status in the 2012 sweepstakes.