The media are awash with tactical post-mortem's of the doomed Rudy Giuliani campaign. The analysts will see the failure of Giuliani's campaign as a matter of strategy, tactics and competence, because that's how they see everything. But the simple fact of Giuliani's fall is that it was inevitable. No matter how well he did in the early, name recognition portion of the campaign, he's pro-choice, pro-gun control, pro-gay rights in a party that defines itself as being adamantly against all of those things. Throw in his highly entertaining personal life and the Republican grassroots distrust of all things New York and Rudy's chances of winning the Republican nomination are about the same as Ted Kennedy's.
Sure, he had 9/11, but it's hard to differentiate yourself on national security when the whole field is in favor of torture, endless war and the limitless power of the Commander-in-Chief. (OK, there was some slight shading. Ron Paul was against a couple of those things, and McCain is against torture at least some of the time.) Giuliani's national defense shtick was the same as everyone else's, so he had nowhere to go. He never had a chance.