Former acting FBI director Andrew McCabe said in response to my reporting that
had he known Mueller was not conducting the counterintelligence investigation into Trump, he would have pushed for the FBI to do it. He gave me this statement.
https://twitter.com/nytmike/status/1300107673015406594

Edited to add:
Deputy AG Rod Rosenstein concluded the FBI had not cleared the threshold needed to conduct a counterintel investigation into the president's ties to Russia. Rosenstein believed the FBI had acted far to hastily to open investigation as it grieved firing of director Comey.
I think we're going to have to wait to see why Muller didn't investigate instead of jumping to conclusions. We are only starting to get the story.