The biggest challenge for an anarchist system of power is survival, and not being overthrown by a competing hierarchical system.
Creating a new anarchist system in a world that has other hierarchical power systems becomes exponentially more difficult the more hierarchical power systems there are surrounding it: as he mentions, hierarchical power systems tend to cooperate to create more concentrated bases of power. The easiest targets for creating concentrated power is where there currently isn't concentrated power, so hierarchical power systems tend to target them first.