Trump only re-appointed Rosenstein because of right time, right place. Rosenstein is a Justice Department lifer (Deep State guy), Deputy AG, who was the AG #2 to Eric Holder, on Obama's watch. When Holder left, Rosenstein filled the vacuum and inherited the interim AG title until Sessions was appointed. Then, almost immediately after being appointed, Sessions recused himself from all the Russian investigations, essentially making Rosenstein the acting AG again.