The reason is obvious, isn't it? After all, it's much more difficult to stoke widespread anger and discontent about our troops serving in Iraq if the successful end of that service is somehow in sight. If troops can come home because the mission has worked, it also makes it much harder to argue that the surge failed -- as Barack had predicted.
Even so, it's revealing of his character that he'd try to demand a delay in troop withdrawal agreements for his own political benefit. What's more, his latest proposed withdrawal date, 2010, would be impossible to meet if agreements were delayed as he requested.
But what does he care? He'd be President by then . . . or not.