And because of racial covenants that banned non-whites from even purchasing certain properties. When a new housing tract/neighborhood was being built in a suburban area, city planners would advertise it as a 'restricted' area - meaning no non-whites would be allowed in that neighborhood. Which was attractive to white people, of course.

The way they did this was by putting clauses (covenants) in property deeds that clearly explicated 'This property should at no time be occupied by those not of the Caucasian race'. The FHA would refuse to subsidize any landlord that dared to sell one of these properties to a nonwhite. This language can still be found in deeds all across Massachusetts, and this nasty strategy was used in tandem with redlining.