Because of good job opportunities and better public resources. For one thing, a large number of ordinary people tend to live in the city mainly because they want great career, it seems much easier to hunt a decent job in the city, that is to say, those who want big money certainly would live in the city. For another, in the city you would probably find lots of educational and medical resources as well, or let us say, other kinds of better resources in general. Like the top universities for example. Living in the city could make people have more choices. That is it.