@Paradox25 I think McCain had to pick a wild card, because he was on the losing trend already. In 2008 the flow was heavily against the Republicans. McCain tried to pick Palin as a Hail-Mary-Pass of sorts.. and it failed (somewhat predictably).
This year though the tides have turned, at least somewhat. The flow is much more even, or even against the Democrats. Thus far in the campaign Romney has been Mr. Reliable. Unlike his foes in the primaries he didn’t take any crazy stances. He’s been party line, and economy, economy, economy. He knows that if the Republicans are going to beat Obama this year, that’s the only real way they can do it. On social issues and the international issues, the public is pretty solidly behind Obama. Their only real strong stance is on the economy. So why would he endanger that with a wildcard pick, which could just as easily go negatively as positively?
@SuperMouse I doubt Christie would take the nomination, as I feel he is gearing up for his own presidential run in 2016 or 2020. Failed VP bids for whatever reason tend to have a poor track record going forward. Rubio would take it, but I doubt he is offered because he is definitely a wildcard.