The Mueller investigation is a threat to the president trying to drain the DOJ and FBI swamp. As long as Mueller is in place, Rosenstein and Sessions remain in place. As long as Rosenstein and Sessions remain in place, the DOJ and FBI swamp doesn't get drained.
A nice-to-have but not a priority. The main reason is that the senior staff is hard to dislodge. Someone trying to do that would end up like Pruitt, smeared with innuendo, set up with stings, lied about, etc.