Why?  Because Whitaker hasn't done anything and everyone knows he's a temporary placeholder.  I assume Trump wants Barr for a reason.  A better question is why would Barr want to be part of Team Trump?  He would go from a comfortable and respected position to one of intense scrutiny, partisan bickering, basically his life turns upside down.  Who would want this?  Why would anyone want to take a position they already had in the past with a president as contentious as Trump?  Especially when they don't really know each other?  Hmmm.  There must be more here than meets the eye.