They were too afraid to challenge the "religion of peace" and stand up to Sadr's extremism. Same thing happens in the UK. How many years did that hook handed, hate preaching Imam have free range to preach his crap right out in the open? Sadly, there are hundreds more just like him.
Iraq was largely secular, even under Saddam's dictatorship. Only in Saddam's last decade did he start inviting the fundamentalist in to set up training camps and infest small villages.
When Saddam was taken out, there was a mad rush of fundamentalists to get into Iraq and establish hard line Islamics, knowing full well that Westerners, not very educated as to what Islam really is and could be, would be too afraid to stand up to that religious cults extremism, too afraid to blow the heck out of a mosk used as a terrorist base, too afraid to shoot a black robed devil.
Why? Do you think the Iraqi army went and trained itself?
And do you think all those Afghan artillery men in Helmand province went and trained themselves?
We (the US and UK) adopted a policy of leaving Sadr for the Iraqi army. No-one on FR liked it, and I'm still not sure about it. But Sadr's father was a saint and martyr to the Iraqis for standing up to Saddam: it would have been perilous for foreigners just to off his son.
Now finally the Iraqi army, with its UK and US MITT embeds, is kicking militia-butt and FR's most ignorant are blaming some of the people who made it happen.