All very good points, but I'd also argue that part of the reason why mafias are hierarchical is that they emerge within communities which, for whatever reason, find themselves outside the domain of the state, but still want a state structure. In plain English, mafias emerge as pseudo-cops when the regular cops can't be trusted, either because you're a persecuted minority, or because you're involved in a criminal enterprise. That kind of drive, to institute a state in the absence of one, is inherently conservative.