Nobody says it was; such disasters are often multifactorial. But given to his position that person holds key knowledge and insights into what went wrong that no-one else has. Without access to that information, investigators can only hypothesize.
This is why things like whistleblower laws and indemnity insurance exist, to enable the full and unvarnished truth to come out. How are errors meant to be fixed correctly when you don’t have all the information as to the cause?
Compare how air crash investigations work. Or research into procedural improvements to hospital hygene. There are things far more important than just finding people to blame.