I think it is mostly due to population density.

If the number of asshats is evenly distributed (like, say, 1 in 100 people is true asshat), you will encounter more of them per square-kilometre in a higher density city.

If the number of asshats is evenly distributed (like, say, 1 in 100 people is true asshat), you will encounter more of them per square-kilometre in a higher density city.