The problem is if you are a criminal you need to cover your tracks and your client tracks. Someone can meet with an assassin to kill so and so on a street corner. However how do you get a large amount of cash to the assassin if they pull the murder off. Large amounts of cash are too often tracked, and most other electronic payments are as well. Bitcoin has a public ledger that is not as anonymous as advocates like to claim (and the police are presumably getting better at understanding it)

I don't know if a contract is a good way, but it gives some deniability.