Because he knows whatever gets released to Congress eventually get leaked to the public.
Barr is going to follow the letter of the law, and everyone needs to understand that some information is kept confidential for a very good reason. I’ll once again post Rosenstein’s letter from last year. It was very clear in explaining this and emphasizes that the DOJ has to regain public trust in the institution
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I would encourage all anons to read this so you can explain this to people that don’t understand.
https://www.justice.gov/ola/page/file/1080041/download