It took years for the Commercial sector to get around to investigating air incidents properly (and even now you get accusations of hidden/altered evidence)
The military SO's hate the idea that any decision they make /made will be questioned - they see it as the slippery slope to non-obedience. Combine this with the UK Civil Service mania for secrecy and never attributing blame for any past decision and we are left with a poisonous mix
Regrettably I think a constant series of re-visited BOI's will be a running sore for everyone - better to have some outside agency to have the power to revisit old accidents and publish the historic facts