It's the population density. The larger the group there is, the more demand to make and follow rules. Imagine a giant, multi lane turnpike in a small town of about 5,000 people. It's too expensive and useless in an area with one major road. However, the same thing is needed in a city. Everything works the same way, the larger the city, the more complex and thus demanding it gets. Insert the wrong people and it's a dictatorship waiting to happen.
It's the population density. The larger the group there is, the more demand to make and follow rules. Imagine a giant, multi lane turnpike in a small town of about 5,000 people. It's too expensive and useless in an area with one major road. However, the same thing is needed in a city. Everything works the same way, the larger the city, the more complex and thus demanding it gets. Insert the wrong people and it's a dictatorship waiting to happen.