One of the key differences between man and other animals is other animals adapt to their environment while man is unique in often adapting his environment to himself. It's a paradigm shift that can play havoc with adapting evolutionary models within anthropological frameworks.
Once you take into account the environmental engineering that man does traits lending towards social integration becomes obviously advantageous. The traits which allow the cooperation requisite for molding an environment more conducive to the species lead to their selection as the victory of one polis over another. The more cooperation the more the environment can be molded. This includes the citizens as elements of the environment. And so man's ability to adapt his environment to himself leads to the creation of social structures as the most effective, and therefore the most often selected, to realize this. In turn, the strongest social structures are those that allow the greatest number of citizens to realize the maximally socially productive potential. Inclusion is thus selected over exclusion.
Of course this is just my theory and is conjecture. I welcome any counterexamples.

Edit:
Thanks to everyone for pointing out other species that do this. I do think, however, that the way we alter our environment cooperatively (different from beavers) and that the cooperation is conscious and intentional (different from ants) might be singular.