Special interest groups would want to collaborate so that their "special interests" are met, like raising prices. In a world where there is no regulations(aka an organization with a monopoly on violence) a new interest can always provide the same service cheaper(since prices were artificially inflated). There is a natural incentive to not cooperate, as Game Theory would dictate, someone WOULD take that up if there was no monopoly to violence.

This works because there is a natural incentive for ALL service provider AND CUSTOMERS to cooperate, which the SIGs were not doing.