"The Haditha incident, in which US Marines are alleged to have killed between 14 and 24 civilians in cold blood, is becoming the My Lai of the Iraq War. Officers have been relieved of command, and murder charges may be brought. Somehow, though, this time the American public doesn't seem very interested in the story. My guess, is that many still have payback for 9/11 in their minds. The Vietnamese had never done anything to us. Of course, the Iraqis hadn't done much to us, either, aside from fighting back when the United Nations pushed them (quite rightly) out of Kuwait. But Dick Cheney has by innuendo and half-lies managed to convince the American public that in fighting the Iraqis, we are fighting the people behind 9/11, or at least people very like that."
I think the much more mundane explanation is that most Americans have mentally pegged Iraq as another Vietnam, and so it takes longer to react when some new height is reached. People also might be afraid to be outraged at the military - or even a small portion thereof - in the days leading up to Memorial Day.