There is a long tradition of presidents appointing loyalists as Attorney General. Robert F. Kennedy and Griffin Bell come immediately to mind as the brother and childhood friends of John Kennedy and Jimmy Carter. Professor Jed Shugerman has critically explored that history at length in a work in progress.
But there was an important post-Watergate shift which sought to buttress the independence of the office of the Attorney General and the entire Department of Justice from the presidency. In 1977 the passage of 28 USC 508 governed succession in the Department of Justice in the event of a vacancy of the office of the Attorney General – head of the Department of Justice, itself created in 1870, a subject explored in the Stanford Law Review by Fordham legal historian Shugerman. The 1977 Act was preceded by the 1976 Omnibus Crime Control Act 42 USC 3701 that created a ten year term for the Director of the Federal Bureau of Investigation who must be confirmed by the Senate. The clear policy thrust is to strengthen independence of the Department of Justice and Attorney General. It is that which is threatened by Mr. Trump’s appointment of Mark Whitaker as Acting Attorney General to assume oversight of an investigation of the President himself. I discuss the legality of Trump’s appointment in the post linked below- GWC