I think bitcoin is still not so easy to use and not all ordinary people can use it. And criminals digital currency will only interfere and, it seems to me, it is much easier for them to use for their illegal transactions conventional cash.
I think bitcoin is widely used by perpetrators of crime, because of anonymous factors. However, not all criminals use bitcoin because it is not easy, where bitcoin will also be tracked where it flows, unlike fiat money which can be used immediately. So criminals who use bitcoin are criminals who master technology.