My opinion is that it's been more than 40 years since he died and most of the suspects and/or witnesses are dead. All they have is an autopsy report that might not be thorough enough or even accurate. It would be really hard to prove anything with this many gaping holes. What amazes me is that no one came forward before this time and tried to do something. It is for this reason that I think, whoever did Brian in, was extremely dangerous and scared many people into staying quiet or disappearing.
It's sad that Brian will not have justice. Personally, if I were someone who knew him and claimed to care about him. I would not have been able to live with myself knowing I did nothing to find the truth or didn't speak up if I knew or suspected something. I would rather face danger than live with the guilt of knowing I let my son/lover/friend down.
In Hotchner's book, Linda Lawrence's parents claim that Brian was not a good friend to others so he had no real friends. They say he gave the impression that he needed nothing from anyone. They say that Linda was a good friend to him, but no one else. That's a lonely life. I understand why he was sad. Brian is a tragic figure, in many ways.