Pat Riley once said in an interview that he does a much better job as a GM then when he was both coach and GM, it's not the skill or experience, but when it comes to personnel moves Riley the coach fights Riley the GM and because Riley the coach has too many blind spots as he was too emotionally connected to the players and coaches on a daily basis that prevented him from pulling some triggers he should have.
We all know JB is loyal. He may be too loyal to his assistants, he may also believe that the best way to move forward with least amount of disruption is to hire internally so as to preserve his zone defense and string bean forwards. He has blinds spots that JW does not.