I don't know what you're referring to, but I'm curious as I moved to Chicago relatively recently. Could you elaborate?
Most likely: racism/segregation.
Chicago was one of the places restricted covenants were first struck down in the US, so the city political machine used a combination of zoning & public works (notably highway construction & public housing) to keep the city segregated and to limit the political power of Black people.
Wild how often a viaduct or train track separates a Black neighborhood from a neighboring non-Black neighborhood.