One of the reason why Kandahar can not be used as an altenate for Kabul is because to be accepted in Kandahar you need a unique ISF number and transponder code that gives you the right to land there and these numbers are valid for only 30 minutes.
These numbers must be requested in advance and , I guess this takes some time which will render the access to Kandahar almost impossible in case of diversion.
On top of that , if I need to divert going to Kabul I would prefer to go anywhere else than Kandahar...