I believe Sessions probably cut a deal to recuse himself on anything connected to Russia and Clinton/election matters in exchange for being confirmed. He probably also agreed to recommend that Trump keep Rosenstein and Huber.
I also believe that Trump is building congressional support so that he can finally clean out the FBI and DOJ.
Jeff Sessions himself picked Rod Rosenstein on the recommendation of Sessions’ long-time pal Chuck Cooper (of Cooper & Kirk). Cooper is considered one of the top few litigators in DC. Rosenstein was also promoted by Chris Christie.