Key point: the northern tribes are completely different ethnic groups, and some of these tribes (e.g. Hazara) are Shia. The Taliban is a minority in the Pushtun.
(the Uzbeks are Turks, the Hazara are Mongols and the Tajiks are, like the Pushtuns, cousins to the Iranians, Pakistanis and northern Indians). Thus no matter how successful the Taliban might be in the south, among their fellow Pushtun (many of them anti-Taliban), they still have to face "them"; the northern tribes, who now have powerful foreign allies, a combination that proved invincible in 2001, and can do so again if called on. Without their American allies, the Pushtun believe they can, as they usually do, intimidate the more numerous and divided non-Pushtuns into compliance with Pushtun domination.