I think they kept it at a minimum to avoid drawing too much attention and hurting their other rackets. The Outfit had the entire Chicago area to themselves and there was so much money to be made, they didn't have to get into drugs as much as the NY that had over 1000 made guys. Another reason was that Chicago wasn't on the coast with a huge dock like NY. The NY families could control most of the heroin importation from the Middle East because it came through NY docks that were controlled by the mobbed up unions. It's harder to put tons of heroin onto airplanes and fly them into Chicago.