I think people in social sciences tend to ignore the most obvious factor: military power. A successful campaign needs rapid coordinated efforts, which are best delivered by hierarchical models. And if you suck at war, chances are that your civilization won't stick around for very long.
This may be an argument for the superiority of militarist states (e.g. Assyria, Prussia), but it's anachronistic for the study at hand, which deals with the type of situation prevailing in e.g. the pre-civilization and pre-organized army Neolithic.