And one good guess I've heard is because Rosenstein wanted Mueller as FBI director instead of Comey, because they had worked together on Uranium One etc. When Trump turned Mueller down, the very next day Rosenstein appointed him to investigate Trump.
” A new DAG would even have the ultimate, er, trump card: She could decide at some point that the investigation should not even continue at all...”
Judge Pirro perhaps?
Statement by Deputy Attorney General Rod Rosenstein on Anonymous Allegations
Deputy Attorney General Rod Rosenstein today issued the following statement:
"Americans should exercise caution before accepting as true any stories attributed to anonymous 'officials,' particularly when they do not identify the country -- let alone the branch or agency of government -- with which the alleged sources supposedly are affiliated. Americans should be skeptical about anonymous allegations. The Department of Justice has a long-established policy to neither confirm nor deny such allegations."
As soon as I read this, I figured Rosenstein was sending a message to the Trump team to ignore the 'fake news' reports about the Special Counsel investigation that were coming out on a daily basis.