There was one main reason why the inner cities declined. This was the rise of the suburbs. As the suburbs arose, the cities declined.
Before World War II, there were few suburbs and everyone lived in the cities. The cities, therefore, were composed of people who were rich, middle-class, and poor. They were also composed, in many places, of African Americans and whites. The cities, then, were rather prosperous.
After World War II, this started to change. For various reasons, suburbs started to boom. However, the people who moved to the suburbs were typically middle-class or better and were typically white. Only people who had enough money could afford to buy the houses. Companies typically only lent money to white people to buy the homes.
What this meant was that the cities were left as the home to non-whites and poor people. This was very bad for the cities. As more white middle-class people left the cities, the cities declined more and more.
Thus, the rise of the suburbs took away prosperous people from the cities. This is what led to the decline of the cities.
Inner cities were developed before the Second World War and hosted diverse people in terms of race and social class. The cities also served a variety of purposes, such as hosting government institutions, industrial plants, commercial centers, retail areas and also provided housing to the people of that particular area.
After the war, the inner cities went through a period of decline. The cities started to lose their luster and suffered increasing problems related to poverty among a section of the population. The cities experienced a surge in crime, drug abuse, and unemployment. The situation led to an exodus of the upper and middle classes to the growing suburbs. Businesses also shifted to the suburbs further compounding the problem taking root in the inner cities. In order to facilitate the growth of the suburbs, sections of the inner cities had to be brought down to provide space for infrastructure and utilities serving the suburbs, which led to the destruction of established neighborhoods.