It's because in Chicago (and every other city in America) there are far more poor black people than white people, so when a black person attacks a white person, it's usually motivated by economics. On the other hand, if a white person attacks a black guy, it's probably not because he's trying to rob him but because of racism.
Such crimes are pretty rare, which is also why you don't hear about them compared to black on white crime. Most of the crime is in fact done by blacks against other blacks, but you don't hear about that on the news as much either, unless it's a particularly sensational story, because the media is geared towards advertising and once again they target white people because they have more money and the best way to make money off of people is to scare them- don't go in the city where the scary and dangerous people are, move out to the suburbs where the new expensive houses and nice shopping malls are!
it's all about social control and property values.