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.
While this is true, I'd like to expand a little bit on it. It's easy to track when big chunks are moved as a whole, someone that splits it up and buys goods with bitcoin directly would be more difficult to track, though not impossible. Because of this its a bit difficult to prove that the person cashing out is the same person that requested the ransoms, of course it depends from case to case.
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.
Now you do mention something important, but all things considered, would cash then be approved if it weren't already a commonly accepted mean of transaction? Bitcoin at least leaves trails that can be followed all the way to the start. Cash can change ownership sooo much easily, I just don't see how it isn't a counter argument against official currency alternatives, that's all.