"Collusion"..that, and a dictum by AG Bill Barr that DOJ policy precluded Mueller from bringing charges against a sitting President.
Here's an excerpt from the attached Harvard Gazette article discussing this...
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As expected, the 448-page report confirms that Mueller did not find sufficient evidence that the president or his campaign associates coordinated with Russia’s election interference effort. But contrary to Barr’s assertions that Mueller was unable to reach a conclusion on whether President Trump obstructed justice in trying to stop the probe, the report actually says the special counsel felt bound by Department of Justice policy that prohibits bringing charges against a sitting president and therefore did not try to reach a prosecutorial decision.
With surprisingly few redactions, the report reveals a dizzying roster of Russians and Americans involved in the Trump campaign who actively connected, talked, and then met with one another starting in 2015 (although Trump, his attorney Michael Cohen, and Trump’s adult children were in communication with Russian officials as early as 2013 to discuss the Trump Tower Moscow project). The richly detailed report also chronicles Trump’s activities once he became president and how he repeatedly sought to defuse or derail first the FBI’s counterintelligence investigation and then Mueller’s probe, which began in May of 2017.
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Ask yourself, Why would an innocent man go to such lengths to Obstruct Mueller's investigation? Then factor in the fact that Don Jr.'s emails show him to have lied about the purpose of the Trump Tower meeting with Natalia Veselnitskaya in the Senate Intelligence Committee Report. The answer will require you to use your own logic. I'd like to hear it.
Link: https://news.harvard.edu/gazette/story/2019/04/harvard-professor-mueller-found-substantial-evidence-of-obstruction-of-justice/