Rudy's ambition since 9/11 is to make a run for the presidency. Note the absence of interest for Governor or Senate. Could it be Rudy didn't want a track record that could be used against him in a presidential race? It's hard to predict how Rudy would govern nationally without a national or state track record. We know Rudy's record as a Mayor and for many issues it isn't stellar.
Way back in 2001, I remember a couple of posts on FR talking about him being a candidate in 2008. (Not that Rudy had already said he'd run, but that some were saying he'd be a good candidate.)