The second reason for people leaving cities was because of a sharp increase in crime, especially violent crime. The political leaning of the 1960's and 1970's were to be very soft on crime and that era is marked by a stark rise in crime. People that can afford it will not live in a high crime area - go figure! They move to an area that is nicer and the politics of those areas tend to be protective in that they don't want the same political policy that allowed crime to soar.
This of course starts a chain reaction where there are better living conditions and lower crime outside the city and the city begins to rapidly lose its tax base. The entrenched politicians are more concerned with having power than managing a city and the city rots. So they scapegoat people for being "racist: because they didn't want to live in a crime infested, cramped, hellhole when there were viable alternatives. The market speaks.
The main reason people left cities was because of better living conditions in suburbs and the rising tide of high crime in cities. That's it.