Now the problem is that while the Afghan government is installed and supported by the US, it is the eyeshore of the Taliban which rejects the Ashraf Ghani government as illegitimate and considers it American stooge and hence unacceptable. This makes things a bit too complicated.
Firstly, Afghan government's participation in any peace process is necessary to give it a legitimacy, and hence it cannot be bypassed. But to get the Taliban to agree to hold talks with the government is also very difficult because in that case the Taliban has to accept it which it is averse to do. The US is in a fix for it cannot ignore its own government and it cannot ignore the wishes of the Taliban.
So a cautious US is reluctant to withdraw all its troops because that means it will be Taliban all the way. But the peace process cannot move significantly until the US pulls out because that is what is Taliban's first condition.
To add to the US woes, Afghanistan is going to the polls in July. How the Tliban takes the polls is a general concern . The US wants a peaceful election. Chances are remote that the Taliban will keep quiet before or during the polls, simply because remaining quiet may mean it is endorsing the democratic process and acknowledging the government in a roundabout way. So the first priority of the US, according to Zalmay Khalilzad, US Special Representative for Afghanistan Reconciliation, is to have a peace deal with the Taliban before the polls. Troop pull-out can take a back seat for now.