[onepointsixC]

They were able to make the gains they could because the of the low national unity and moral of the ANA. The government that the US built was made up of tribal leaders who were corrupt and self interested with little real unity.

Just compare the incentives. If the Taliban win, Taliban fighters get to plunder cities, forcibly take wives, and get power. If the ANA forces win, they keep a nation which they don't identify strongly with free. Now add in that the ANA is completely demoralized from loss of all US support and which do you think is more willing to fight and risk their lives? It's little surprise how things are turning out.