Mueller was the figurehead. He of the impeccable reputation but Weismann ran the show.
Why did he name an incompetent person to be the head of it, and then keep that person in place throughout?
The Democrats' first "own goal" was insisting that Mueller testify before Congress. This had the unintended effect of showing the American people how diminished Mueller had become, which tore up any credibility that the "Mueller Report" might have had in making charges against President Trump.
Rosenstein must have known and said nothing, keeping Mueller in place for his reputation, if not his competence.
-PJ