air crash investigation

I've just watched a documentary regarding investigation into the SilkAir crash in Indonesia in 1997 involving a Boeing 737. The plane enroute Jakarta to Singapore crashed into a major Indonesian river without any warning or indication of problems on board, killing all on board.
The investigation handled by the Indonesian NTSC reported after 3 years of investigation that they were unable to find any reason for the crash.
The USA NTSB which conducted a parallel investigation were serverely critical of the Indonesian investigation as they had concluded that it was a result of pilot suicide.
After the release of the reports, a lawyer in the USA acting on behalf of relatives for some passengers conducted his own investigation as he had noticed a similarity between how the SilkAir 737 had crashed an d 2 other 737 crashes. His investigation led to discovering a fault in hydraulic control valves used on the rudder and a court case involving the hydraulic valve manafacturer that resulted in a large compensation payout to relatives of those killed.
What disturbed me was the position taken by the USA NTSB which seemed too willing to pin the responsibility on the pilot.
I am left wondering whether this come about because the NTSB investigators were incompetent, or did they allow themselves to be blinkered once they found some unusual aspects of the pilot's life, or were they simply corrupt and trying to protect USA business interests.