I think, and correct me if I'm wrong, the main reasons are:
1) Rap/hip-hop is "protected speech" to them (meaning absolutely sacrosanct)
2) The illegal stuff's already illegal. (It was already illegal for Puffy to shoot up a Manhattan nightclub, for thugs to shoot Tupac Shakur twice, and for more thugs to murder Christopher Wallace, etc.) The socialists have to target freedoms, not criminals.