As best as I can tell the Mueller team is investigating why Hillary Clinton lost the election. All the so called experts said she couldn't lose despite being a horrible candidate and living on the wrong side of the law her entire adult life.
But the experts couldn't have been wrong so a special investigation was warranted. Among her every changing list of reasons for losing, such as stupid voters and a corrupt DNC, the Russian boogeyman was tossed out early as one of the excuses by Clinton herself. Evidently the Special Counsel decided to pass on investigating stupid voters and the DNC.
Once Mueller selected the Russians from the grab bag of excuses for Clinton's election loss there were choices to be made by the investigators. The doctoring of electronic vote counters was ruled out since pretty much everybody else but the Russians perform that neat little trick every single election. And examining voter roles is the third rail of politics since it involves all 50 states and most of them are as dirty as a pig in slop and the states would fight tooth and nail to protect their ace in the hole. The investigators needed something to grab onto that was less obvious, less tangible, and could not be corroborated by cold hard data.
So they settled on the Manchurian Candidate angle. Not only would the press eat it up by the bucketful but the boogeyman in the sky angle required little more than innuendo and American's natural distrust of the Vast Red Wing Conspiracy. Grabbing at ghosts beats the hell out of sweeping the dust from under the rug of the American political system.
