Because he doesn't give a rip, I'm guessing

Foreign Policy: "Why President Obama rarely talks about the war"

And part of the answer is that Obama has thoroughly lost his base on Afghanistan. It is not clear that the left truly believed in the war in 2008, but it is absolutely clear they do not believe in the war now. The left has not yet mobilized against the Afghanistan war the way they mobilized against the Iraq war, and if Obama wins a second term perhaps they won't (if Romney wins, I expect anti-war factions to regain some of their 2006 mojo). The Obama campaign has put all of its 2012 electoral bets on a base mobilization strategy, and so the last thing the president wants to do is remind his base of anything he has done that they don't like.

For all these reasons, and perhaps others, President Obama has largely shirked the traditional commander in chief duty of mobilizing political and public support for the wars he is leading. In contrast with President Bush, who clearly believed in the wars he led and sought every opportunity to try to rally the public to the war cause, President Obama seems far more ambivalent about some of his war duties.

"Ambivalent?" I think that's being highly charitable. I think he simply does not care about the outcome there any more. He doesn't think the United States can win there, in fact he would not know how to define "win" in any event, and so will proceed with vacating the place by 2014. And one outcome will be as good, or at least no worse, than another.