"After reviewing the Special Counsel's final report on these issues; consulting with Department officials, including the Office of Legal Counsel; and applying the principles of federal prosecution that guide our charging decisions, Deputy Attorney General Rod Rosenstein and I have concluded that the evidence developed during the Special Counsel's investigation is not sufficient to establish that the President committed an obstruction-of-justice offense," Barr wrote in the letter.
Barr said part of the reason Trump shouldn't be prosecuted is because there was no evidence of a conspiracy. Since there was no underlying crime that Trump was trying to conceal, proving obstruction would be difficult.