The reason being that they cant intentionaly put the population 'to the sword', because if they did the whole point of their invasion would be lost. On the other hand the Azeris would have no such qualms, their intention would be to 'cleanse' Artsakh not to 'liberate' it. Guerrilla warfare thrives in an unfriendly (to the occupier) populance, when the populance is wiped out there is no place for insurgents to hide.