Why do Republicans nominate the guy who came in 2nd place the last time around for President?

Lets review: 2008 (McCain 1st, Romney 2nd), 2000 (W. 1st place, McCain 2nd), 1996 (Dole 1st, Buchanan 2nd), 1988 (H.W 1st place, Dole 2nd place), 1980 (Reagan 1st place, H.W 2nd place), 1976 (Ford 1st place, Reagan 2nd Place), 1968 (Nixon 1st place, lost by an inch against JFK in 1960). My question to you is why does the GOP do this? Except for 2000, we literally have seen the GOP go with the next-guy-line candidate. Can you explain to me why, without resorting to partisan slams?