The Taliban WERE defeated and expelled from Afghanistan. They no longer impose their notions of strict Islamic Law on the people of Afghanistan.
The Taliban slipped over the border into the mountains of Pakistan where they were welcomed. Those tribal areas are fiercely independent, and the Pakistani government has almost no control over the area. The radical ideology of the Taliban and their hosts remains a danger to both Afghanistan and Pakistan. The U.S. government has been reluctant to carry out operations across the border into Pakistan. We are in a balancing act between hitting the Taliban's sanctuary hard, and preserving the soveriegnty of the Pakistani government whose assistance we require. As the Taliban has increased its attacks across the border, this administration has increased the number of strikes into the Tribal lands of N.E. Pakistan. That is hurting the Taliban, but at the same time it undermines our efforts to keep moderate Pakistani governments in power. A significant portion of the Pakistani population has always been supportive of the Taliban's radicalism, and the ISI was one of their main sponsors.
In the meantime, the U.S. has strongly supported improvements inside Afghanistan. The completion of the Ring Road should help, and our support of education, health, and other public works is money well spent. Still, there are major concerns. The central Afghani government isn't as stable as we'd like, and far too much of the country remains the province of local warlords. Exports of opium are up, and any effective attempt to suppress the trade might further erode support for the existing government. Putting boots on the ground and providing logistical support to Afghanistan is difficult, so we've placed probably too much emphasis on air power and the use of armed drones. It seems a relatively inexpensive means of countering the Taliban and Al Queda's efforts to return to power, but in the process we're alienating a whole lot of Afghani's who end up killed or wounded in the process. How many innocent Afghanis have been harmed, no one can say for sure because the radicals don't wear uniforms and a fanatic's corpse looks exactly like the corpse of an innocent villager who would just like a bit of peace and quiet.
The Taliban WERE expelled from Afghanistan, and if our forces were as evil as the Left would like to paint them, we might have cleaned out that rat's nest a long time ago. Or maybe not.