Because of the semi-anonymous nature of Bitcoin, its used in ransomware attacks. Hackers breach networks and hold them hostage until payment is made to them. Hospitals and schools have been victims of these types of attacks. However, unlike cash, which was favored by thieves in the past, Bitcoin always leaves a trail in the blockchain that investigators can follow.
It is becoming relatively easier to track bitcoin transactions especially if they eventually cash out through exchanges which normally would require KYC procedures of the user.Bitcoin is certainly a tool of choice to extort people who want to remain in the 'dark'.This is the main argument against adoption bitcoin as a legal tender especially from government circles.
unless they could cash out without going through exchanges.