The collapse of the blue collar put millions out of jobs. Gary, Chicago, Detroit, Cleveland ect were all built upon industrialization as well as the Great Migration hence the large number of blacks in these cities.
Crack hit these places harder as well due to the already dwindling incomes of the residents and thus the cheapest drugs were more prevalent.
Lastly, these cities only have about 4 months (May-September) of true summer, as some one said thats only 4 months to be outdoors on any sort of consistent basis, thus the streets move 5x as fast in the summer. Its not New York where a million nikkas are doing shyt at any given time. So that gives you less than half a year to get it in.
Pre-Katrina New Orleans i never really understood.