The article at the link explains the 4 key drivers of the reduction in the 1990s:
1.) Increased # of police (Costs $’s)
2.) Increased incarnations (Costs $’s)
3.) Reduced Crack use
4.) Legalized abortion in the 70s leading to fewer criminals (costs souls)
It also covers factors that didn’t have an impact like new policing strategies
Link: http://pricetheory.uchicago.edu/levitt/Papers/LevittUnderstandingWhyCrime2004.pdf