A typical rural school attracts students based on proximity---if you're living in that district, you go to that school, no matter how much your parents make or whether they're lawyers or a convenience store clerk. (Additionally, the residents of the area are typically less interested in education, rather than getting a job or playing sports.)

In an urban area, residential areas are typically segregated by wealth, and wealthier parents want their kids to go to good schools. Further, they are capable of moving within the area so that their children go to good schools. As a result, the competition among students for the top N% more fierce due to the pre-selection of students.