It pretty much solely has to do with conservative media wanting to make the president look bad (he served in Chicago so by association the Fox News and Breitbarts of the world cannot report anything about the city that doesn't make it look like a horrible place to live) and due to slightly stricter gun laws than many big cities (a desperate grasp by the media to show that gun control cannot possibly decrease crime). As such, these numbers are reported without any context. In Chicago alone the homicide rate has plummeted in the last couple decades. Nor will you see these numbers compared to any other cities. Current homicide rate in Chicago is 15.2 per 100,000 where as a city like New Orleans is a rate of 57.6 per 100,000.
Here is a list of many of the major cities in the U.S.. Why is it that we only hear of the rate in Chicago. You'll notice a lot of other cities with rates higher. http://en.wikipedia.org/wiki/United_..._by_crime_rate