Romney passed because he would lose. Christie will win in a landslide.
Would that necessarily be the case if Booker runs though? Maybe Christie would win that race now, but 17 months is a long time, and any number of things can happen.
If Christie ends up finding himself in a competitive race, then think about the media dynamics that'll develop. Because everyone will know that Christie is a likely 2016 candidate, he will constantly be asked pointed ideological questions designed to box him in, so that he has to choose between appealing to a 2013 New Jersey general electorate and a 2016 GOP national primary electorate. He can choose the former and win reelection but perhaps risk weakening his chances in 2016, or choose the latter, and risk losing reelection. Is Christie really so popular in NJ that he can risk already having one eye on 2016 while running for a second term that he doesn't actually intend to finish?