Hm. Monopoly on violence in the political sense doesn't mean being the unchallenged masters of the territory - it means that people in that territory can't do violence to each other, or in this context engage in tribal or clan conflict, because the settlement of disputes and the application of law is the remit solely of the governing power. I don't think that's ever been the case in Afghanistan, i.e. even the Afghan state ain't much of a state.