I asked about that earlier but no one gave an explanation. Barr did write that 'memo' outlining why the DoJ can't indict a sitting president (something about being in charge of law enforcement, not wanting to impede his duties of office, plus broad Executive powers). Probably why Trump wanted him as AG.
Since AG Barr would never agree to criminal indictments, the 'policy' perpetuates itself. Mueller seems to be a process & procedure lawyer, following 'guidelines' and rules as mandates. Presidential indictments were never on the table because that's 'policy'.
What's even crazier about this is how Barr misled the public about the report's findings....and even made it sound like Mueller could have gone outside DoJ guidelines if he wanted to. So many contradictions and confusions, what a mess.