That's a very interesting point. But how many times did a candidate who had earlier lost an election even bothered to come back and try again? Nobody other than Nixon comes to mind, but Walter Mondale and Bob Dole are two guys who have pulled off the unusual "exacta" of losing once as a VP candidate (Dole in 1976 as Ford's running mate and Mondale in 1980 on the Carter ticket) and later as a presidential candidate (Mondale in 1984 and Dole in 1996).
Henry Clay ran for President three times--the third time he lost very narrowly and would have won the election if a third party candidate had not tipped New York into the Democratic column by draining off some votes that would have gone to Clay. William Jennings Bryan was the Democratic nominee three times, and 16 years after his third try the Democrats put his brother on the ticket as their VP candidate (in the election in which the Democrats got their lowest percentage of the vote ever, if you exclude the four-way race in 1860 when there were two Democrats running).
More recently, Thomas Dewey was nominated again in 1948 after losing in 1944, and Adlai Stevenson was nominated again in 1956 after losing in 1952. Hubert Humphrey was one of the primary candidates in 1976 after losing in 1968. George Wallace ran again in 1972 after losing as a third party candidate in 1968 (it was in 1972 that he was shot). H. Ross Perot, of course, ran again in 1996 after running in 1992. Ronald Reagan ran unsuccessfully twice before becoming the nominee in 1980. Gore ran in 1988 before being the Democratic nominee in 2000. McCain ran first in 2000 and of course Romney ran in 2008. I think Dole was an also-ran once before he was the nominee.