Deliberately car-centric utopian modernism (the Garden Cities movement, Frank Lloyd Wright’s Broadacre City, etc.) is certainly a factor, but I’d say it’s as much, if not more, about racism. Once de-jure segregation was outlawed, making property expensive by using minimum lot sizes to force people to buy a lot of it at a time became the next-best way to keep (poorer, on average) black people out.