Based on his reading of the Constitution, he could have said: Because a president cannot be indicted, the special counsel’s evidence should be referred to the House of Representatives, which under the Constitutionis charged with weighing evidence of presidential “high crimes and misdemeanors.”
At a minimum, Mueller could in his farewell have paraphrased this memorable statement from his report: “The President’s efforts to influence the investigation were mostly unsuccessful, but that is largely because the persons who surrounded the President declined to carry out orders or accede to his requests.”
But he didn’t say anything so clear or direct. Historians will puzzle over why.
Perhaps Mueller truly thought a more forthright version would be unfair to the president. Perhaps he worried about overstepping the limits of his office. Perhaps his review had convinced him that the president had, as Trump claimed, been unfairly hounded by Congress and the media over Russian collusion, and that any obstructive actions were attempts to protect himself from harassment.