Julian Assange was arrested by the UK authorities and was charged with hacking (!) by helping Chelsea Manning who is in jail for contempt for refusing to testify.
This case has very strong First Amendment implications. The leaker and the publisher. Who is liable? Will it have a chilling effect?
This has also strong European Convention on Human Rights (the Convention for the Protection of Human Rights and Fundamental Freedoms) implications. Julian Assange was accused by politicians of capital crimes, and the UK cannot extradite anybody under those circumstances (see Soering v United Kingdom 161 Eur. Ct. H.R.).
Or are they afraid the Mueller report would get leaked (oh, trust me, if Barr misrepresents, what is clearly doing now, someone will leak eventually leak it)?