In the same way that journalists attribute general election defeats to various quirks of the candidates rather than, say, the state of the economy (see Dole, Bob), there's currently a rush to "explain" Rudy Giuliani's collapse. The New York Times offers this litany:
As Mr. Giuliani ponders his political mortality, many advisers and political observers point to the hubris and strategic miscalculations that plagued his campaign. He allowed a tight coterie of New York aides, none with national political experience, to run much of his campaign.
He accumulated a fat war chest — he had $16.6 million on hand at the end of September, more than Mitt Romney ($9.5 million) or Senator John McCain ($3.2 million) — but spent vast sums on direct mail instead of building strong organizations on the ground in South Carolina and New Hampshire.
...[C]uriously, this man with the pugnacious past declined to toss more than light punches at his Republican opponents.
But as I wrote before, Giuliani is fundamentally unacceptable to the GOP. Like Joe Lieberman in 2004, his early lead in the polls was built on name recognition but he was never going to win. This passage in the Times article comes closer to the real dynamics at work:
Perhaps a simpler dynamic was at work: The more that Republican voters saw of him, the less they wanted to vote for him.
Update 2/1 10:24 AM: The Washington Post has a long account of the tactical failures of Rudy's campaign. The prevailing theme, though, is that he was fundamentally unacceptable to conservatives.