Explain Time Square in NYC then. Most of what was changed there was legal activity, such as porn shops. Why has crime so GREATLY decreased?
For the same reason "good" people act in ways that make a community better, "bad" people act in ways that make a community worse.
I've seen too many good people take a hard drug for some stupid reason, and turn into bad people who would have remained good, if they had never touched the drug.
I'm not a New Yorker (thank God), but I'd guess the proper application of zoning laws and the increased presence of uniformed police officers on foot had a lot to do with it.