The reason is what it did not contain.  Critics had sought the memo as the "smoking gun" to show how former Attorney General Bill Barr scuttled any obstruction charges against Donald Trump.

The staff of the OLC actually found that the allegations did not meet the standard of obstruction even without any defenses or privileges related to Trump's office.

The issue of obstruction of justice ran throughout Barr's second term as Attorney General. Before his confirmation hearing, a memo was released that Barr wrote to Deputy Attorney General Rod Rosenstein on a potentially serious flaw in the use of the most likely federal provision on obstruction of justice against Trump. Barr was hammered by Democratic senators on his view of obstruction, as was I when I testified the next day as a witness. I agreed with many of the flaws noted by Barr in the memo.