Because he sucks. Don’t get me wrong, a lot of what BoJack did was inexcusable but Doctor Champ blamed his fall back into alcohol on BoJack so easily, and even when BoJack took him to another rehab he’s still drinking later on the show. He did not take responsibility for his addiction and would rather blame BoJack for ruining him. Whilst the mix up with the vodka was partly BoJacks fault he didn’t make Doctor Champ down it and evens tayed with him to keep him safe until he sobered up. Then he made the choice to go to a bar.

Also whilst I understand he’s not a real doctor so there’s no confidentiality requirements, it’s a dick move to tell other people what you learned about someone during therapy. It’s like a pretty obvious rule that what you learn in that setting is private. If he was really so concerned about BoJack’s past behaviour he should have reported it to the police, not some reporters.