Quick answer - a lot of it is perceived. In 90% of cases (including Chicago), crime was worse in past decades. People forget or don't care to realize that in cities like Chicago the police do what the fuck they want regardless of who's in charge. A lot of it is easy to look up but people find it easier to take sensationalist headlines and run with it, mostly so they can deride the 'failed liberal policies'. This entire post is a pretty transparent attempt at that, especially since this isn't even related to organized crime.

Also, if you want to see a liberal city that has decreased crime drastically, just look at Boston. But like I said, in many cases the numbers speak for themselves.