Actually, it can be traced back to WELFARE. Welfare created a dependency amongst the lowest economic strata. Back in the 60's during the civil rights movement, white guilt allowed the handouts to become a way of life for blacks. Welfare also encouraged the breakdown of black families (the more kids the bigger the check). When dependent on welfare, its a lot easier to live in center city USA due to lack of car ownership and the convenience that cities offer.
As crime went up in major cities, white plight took money and small businesses out of the cities. The population left behind were mostly government-dependent people. They elected local "leaders" that promised more government programs and handouts - there's nothing like voting yourself a raise and a few more bennies.
Nationally, the democrats latched onto blacks as a voting block. Today we have race pimps Sharpton and Jackson. The democrats as a party have made race baiting and exploitation of the social achievement gap an artform. So if you are in the lower economic strata and are told from the day you are born that your situation is what it is because of whitey, big business, big oil etc., you will vote for the people that allegedly are looking out for you.
Wrong, welfare rolls are higher in rural areas than urban ones.