1. It permits crime to pay. Yes, drugs ought to be legal, but "crime" also includes other stuff that has a victim, such as theft, extortion, fraud, fake products, misappropriation of funds, forced prostitution, etc.
2. It behaves like economic globalization, to the n'th degree. Don't like your local tax regime? Pick one, with the click of a mouse. Right now big business gets expensive tax lawyers to set them up schemes where they are nominally doing business somewhere else, but you can bet their expensive tax lawyers are eyeing bitcoin with interest. So what if the roads locally have potholes and the school got sold off?
As to why it's a new issue, that's just down to technology. It didn't used to be feasible, except by gold-smuggling or the suchlike. That has changed.