It’s a message called, leave under your own power.
Sessions could rope in Rosenstein now for letting Mueller run wild. He hasn’t. He doesn’t have to recuse himself if the subject matter has gone past the original nonsense about collusion.
If he quits and a non-tainted AG can come in to take his place, the entire coup attempt can be constrained to the truth and that means Mueller and his astonishing far left hit squad leave the building empty handed. Hopefully escorted by Secret Service and banned from Federal empployment for life after that, since they were engaged in a conspiracy to overthrow the elected government under color of law.
Why deliver such a message publicly? Privately would work just as well, probably better.