Considering Mueller's direct involvement in the Uranium One conspiracy, including traveling to Russia to deliver a uranium sample (since when does an FBI Director deliver uranium samples to Russia?) on behalf of Hillary, he should have been looked into long ago.
He was appointed by Rosenstein, who was also in on the deal, to keep this collusion from coming to light. Rosenstein is still Deputy AG and will continue to work behind the scenes to make sure Uranium One never gets investigated.