Rust Belt cities like Chicago have declined in population since the mid-1900s and the population loss is not just limited to poorer neighborhoods:

For a long time, most accounts of Chicago’s lagging population have focused on parts of the South and West Sides where many residents, largely African-American, have decided to decamp for the suburbs or the South in search of better schools, less crime, and more jobs.But the under-appreciated flip side of population loss in those parts of the city is that places that ought to be growing like gangbusters are stagnant, often sitting 25% to 50% below their peak populations. Lakeview, for example, was once home to 124,000 people; its population is now 94,000. North Center is down from nearly 49,000 to under 32,000. West Town, which includes Wicker Park and Bucktown, has fallen from 187,000 to 81,000.

What explains the population loss in even popular neighborhoods? Here is one possible answer:

Since replacing a couple two-flats with a courtyard building is now illegal, developers make money by tearing down an old two-flat and building a luxury two-flat in its place. Or they build a mansion, and the neighborhood actually loses a housing unit. As a result, as a neighborhood becomes more attractive, the city encourages fewer people to live there.

Zoning (theoretically based on improving the neighborhood) plus chasing profits may just lead to population loss. This could be balanced out by approving more high-density housing in a particular area (like the Loop are in specific portions of popular neighborhoods as to limit their effect) but that leads to major changes in two places.

It is still worth noting that the areas that seen an increase in population are either (1) the Loop with a reemphasis on residential construction and (2) community areas on the edges of the city which other lower densities as well as potentially more open land since 1950.