From The Atlantic, a far more reliable and thoughtful source than TV news:
https://www.theatlantic.com/ideas/archive/2019/04/mueller-report-impeachment-referral/587509/
"In his report, Mueller took pains to detail why he “determined not to make a traditional prosecutorial judgment” as to whether the president had broken the law by obstructing justice. He began by noting that he accepted the opinion of the Office of Legal Counsel (OLC)—which issues guidance for the executive branch on questions of law—that a sitting president cannot be indicted.
That, Mueller explained, posed an insurmountable problem. A normal investigation would end with a prosecutor deciding to bring charges, or to drop the case. It’s a binary choice. But “fairness concerns counseled against potentially reaching that judgment when no charges can be brought.” Ordinarily, a criminal charge would result in “a speedy and public trial, with all the procedural protections that surround a criminal case.” But if Mueller were to state plainly that, in his judgment, the president had broken the law and obstructed justice, it would afford “no such adversarial opportunity for public name-clearing before an impartial adjudicator.” In other words, because a sitting president cannot be indicted, making such a charge publicly would effectively deny Trump his day in court, and the chance to clear his name.
"
If we had confidence after a thorough investigation of the facts that the President clearly did not commit obstruction of justice, we would so state,” the report explained: "
Based on the facts and the applicable legal standards, however, we are unable to reach that judgment. The evidence we obtained about the President’s actions and intent presents difficult issues that prevent us from conclusively determining that no criminal conduct occurred. Accordingly, while this report does not conclude that the President committed a crime, it also does not exonerate him""
...
"The constitutional mechanism for resolving this situation is impeachment. The president, no less than Calhoun, deserves a chance to clear his name. The public deserves a chance to examine the evidence against him. And his supporters and opponents alike deserve the clarity that only convening impeachment hearings can now provide. "