I agree with everything that Quin wrote, but I would point out that one reason conservatives came to Bush’s defense on Katrina was how over the top, ridiculous, and unfounded the attacks were coming from the left at the time. While people were still in peril in New Orleans, the tragedy had already been co-opted by the left to further their own agenda. Suddenly, Katrina was a racist hurricane, caused by Bush’s opting out of the Kyoto Treaty, compounded by the fact that helicopters were in Iraq. Amid all of this craziness, many conservatives reflexively came to the president’s defense, even though there were plenty of legitimate ways to criticize his incompetent response.
I’d also add that another thing that often isn’t recognized by conservatives is that because Bush was totally out to lunch in the immediate aftermath of the storm, he had to overcompensate later by throwing all sorts of money at the problem, without paying much attention to how it was being spent.