It's not easy for a culturally alien outside power to win the support of a people with a long history of resistance to foreign invaders. It's even harder to win that support after we've spent the better part of a decade proving we don't deserve it.
More U.S. troops are supposed to enhance security for ordinary Afghans, as well as facilitating civilian improvements that will win their allegiance. But more U.S. troops also mean more deaths for innocent Afghan bystanders, not to mention a greater daily irritant to nationalist sensibilities.
The people we aim to help, keep in mind, have no powerful reasons to like or trust us. A lot of Americans feel a visceral aversion to our national government -- regarding it as incompetent, dishonest and overly powerful. So imagine how it looks to Afghans who see our soldiers in their streets. It's no great asset that our chief ally is a regime that had to rig elections to stay in office.
Another obstacle is that the biggest threat to our security and the stability of South Asia lies beyond our reach, in Pakistan. It's hard to battle a foe that can find sanctuary just over the border, where we are constrained in pursuing them. And the harder we fight the Taliban in Afghanistan, the more likely we are to push them into Pakistan, and the more likely they are to assist Pakistan's own insurgency, which is not exactly helpful to our interests.
Getting out of Afghanistan sooner might have consequences we'd prefer to avoid. But escalation offers only slim hopes of averting those repercussions. If we're likely to fail, we can do it after we sacrifice a lot more lives and money. Or we can do it before.