Prosecutorial discretion! 

Paul Rosenzweig carefully analyzes the ambiguities in Attorney General Barr’s letter on the still secret Mueller report which reportedly concluded that the Trump campaign did not conspire with the Russian government to steal data and interfere in the 2016 presidential election.

The second aspect is that despite conflicting evidence Mueller decided not to charge Trump with obstruction of justice.  The key factors are likely difficulties of proof of intent (liars lying), and adherence to Justice Department policy not to charge if there is another remedy – here impeachment by the U.S. House of Representatives. – gwc

Source: OTHERWISE: Unpacking the Barr Letter re Mueller ~ Rosenzweig // Lawfare – Brookings Institution