Because the media would be in there broadcasting photos of children dying from disease and starvation,and people killing their puppies and kittens to eat. It would be a PR nightmare. So would going in and trying to take over by force. It's a no-win situation unless we can manage to bribe the senior Republican Guard leadership to turn on Saddam and surrender the city.
Then they should go with the plan that succeeds and gets the fewest Americans killed.