Why is St. Louis the most dangerous city in the U.S.? Why does it have so much violent crime?

The City of St. Louis itself has many areas with substandard housing, many abandoned buildings, and many, many very poor people. The education system has numerous problems. The major commercial areas and most of the manufacturing areas are just fine during business hours, but after business hours and after dark it is simply a place with far too many gangs and too few police. The city is surrounded by many lovely and safe suburban areas, however, and visitors need to know that!