It needs to tighten around the Special (Deep State) PERSECUTOR and his band of thugs.
Mueller is being used to bring out the Russia entanglements of the Clintons and perhaps the Obamanites, too. It is much more effective if Mueller is forced to be the apparent conduit of it than if Trump partisans and appoiintees were the ones to be credited or blamed with it. Sessions is making this happen but not gelding the results by being the obvious actor.