Congress has the authority to prohibit a President's corrupt use of his authority in order to protect the integrity of the administration of justice
Like I said, Mueller believes the proper place for the use of that evidence is in the Senate during an impeachment trial. He did not subpoena Trump because he already had plenty of evidence. People around him conspired with the Russians plenty, some are in the criminal process for that. Trump apparently did not, other than his stupid statements on TV, mainly because he had people to do stuff like that. The head of the crime family never pulls the trigger himself. I still see RICO prosecution coming when Trump leaves office. Too many crimes to ignore.