This stops being the norm when desegregation occurs. Now that all races must be allowed to mix in schools, businesses and housing, whites flee to the suburbs, put up giant financial barriers in the form of zoning, and refuse to be annexed.
These days overt racial politics is now dogwhistled as concerns about crime or undesirables or culture. But that's a big heaping portion of why things are the way they are.
(This only really applies to suburbs within the same state of the main city; New Jersey, most of which is suburbs, would not face the same annexation pressure since the main cities their suburbs are part of (Philadelphia and New York) are not in their state and can't legally annex them.)