Simple. Take Mao Tsetung's famous phrase, guerrillas are fish that swim in a sea. The sea being the civilian population. Plenty of Afghans know who is Taliban and who is Al Queda. All they have to do is drop a dime, call the Americans or the Afghan police and that bad guy becomes a "detainee" in about 45 minutes. If enough Afghans did this, Taliban and Al Quada would be toast. This can happen. Ask the Iraqis.
Why doesn't this happen in Afghanistan. Fear. Anyone who rats on the terrorists gets killed, and his whole family gets killed too. The terrorists don't do Miranda warnings, they just pull the trigger. The Afghans know this. And, the bad guys will be there forever, whereas the Americans are fickle as the weather, here today, exit planned tomorrow. Any Afghan who risks his life, and the life of his family, by passing intelligence to the Americans might get hung out to dry tomorrow. They all know about the American bug out after Charlie Wilson's war.
The Obama administration's dithering over Gen McChrystal's request for more troops is hurting our cause. The longer the dithering goes on in the public press and on Fox News, the more the Afghans get the idea that the Americans are going to pull out, real soon now. Obama needs to do something and do it now. Longer he dithers, the more Afghans get the idea that the Americans are not serious.