That's why President Trump nominated Rosenstein. Nothing more. Not strategic Machiavellian deep state chess nonsense. Sessions wanted him, so Trump nominated him.
Sessions wanted Rosenstein as his Deputy AG. Trump gave Rosenstein to him.
These are not complex multidimensional strategic decisions.
These are straightforward reasons for what transpired.
Camp Rosenstein - supported by Sessions, Boente et al.
Camp McCabe - supported by Page, Baker, Lawfare et al