More punishment, killing, death and destruction was needed. When you look at WW2 it was only after we utterly destroyed many cities in Germany and nuked too in Japan that the fight went out of these countries and they really, truely gave up on war. You have to make everyone in the country really, really, sick of war. This happens when lots of people you know are dead, houses are destroyed, relatives are gone. Iraqis, even the bad ones, did not have this experience.
We used our advanced technology to have a very "low impact" war when we needed a high impact one. Hopefully the War College planners are studying this and understanding it.