Gangs tend to organize around any kind of illegal, lucrative market. Smuggling (which is really what drugs and alcohol are about), gambling, prostitution, vote buying/selling, theft. Because those activities are outside the law, the law can't really regulate the market.

Then there is racketeering/blackmail, which is probably more of a problem of ineffective policing, at least as far as the "Hey, you've got a nice business here, how about you give me a cut to make sure nobody comes by [sic buy] and smashes your cabbages" variety goes.