They lost what, a battle? A city, for a while...?
There's a problem with seeing these conflicts in terms of win/lose. Al Qaeda, or other terror organizations don't have a central HQ that can capitulate and make sure all its "troops" cease fighting. Even if you could magically make AQ disappear from the face of the Earth today, a few malcontents could reform it tomorrow.
The Taliban in Afghanistan are a good example. They lay low for a couple of years before starting to come back in ever more savage attacks. They can afford to wait, to bide their time. Hell, they live there. Unless they lose popular support and get rooted out by the locals, they can never really "lose," only be kept in check for a while.
So, as AQ really been beaten in Iraq, or are there simply aware the reinforcements will become unsustainable for the US in April '08? That's just a few months away. Why escalate conflict with a much better armed force now, when you can instead keep the chaos at a low sizzle until about 30,000 of the US troops go home?
If the US seems to be making good progress, you might have to escalate against the larger force, but right now, the political aspect of what could eventually become a working solution is in shambles. An Iraq without a working central government is just teetering on the verge of chaos, there's very little to do to push it there.