It's a good question and Les Gelb, via Joe Klein, offers a good answer:
"They simply won't do that. If you stand down, you allow the enemy even this inept Afghan government to create a bow-wave effect, to create the impression of authority and security. The Taliban aren't stupid."
Joe's piece is a superb tour d'horizon of the decision. He wanted more derring-do and passion in the speech, as did Matthews, O'Reilly, et al. I just find it hard for someone inheriting a war already eight years old in the economic straits we are in to give a Henry V address, as if we did not know the immense and complicated difficulties of carrying on a war solely to prevent a single terror attack that might come from somewhere else anyway and needs no real weaponry to succeed.
Obama couldn't fake a conviction he doesn't have and that few adults at this point could truly feel. What he has done instead is replace rhetorical drama with an influx of troops so swift and so large it could alter the dynamic on the ground and give us one more chance to break al Qaeda's back before a withdrawal in our long-term strategic interest. The speed of the deployment (and the CIA's work in Pakistan) is what Obama insisted upon:
The real haggle was over speed of deployment.
The military plans carefully, in five- to 10-year increments, and moves with the speed of a supertanker. A good part of the reason the troops were sent to Helmand instead of Kandahar, even though it violated the prevailing counterinsurgency strategy, was that the fortifications already had been built in Helmand; it seemed too late to turn the supertanker around. Obama kept sending plans back to the Pentagon, seeking a faster launch for his "extended surge." The military still isn't entirely sure that it'll be able to move 30,000 troops to Afghanistan by August. "We'll push in every way possible to get the forces on the ground ASAP," a senior military official told me. But the President clearly believes that the speed and vehemence of the new offensive will be its greatest assets.
Give him a chance. But hold him accountable.