First, that's 121, not 135. Second, what makes this   

Nothing in the NTSB report showed any violation of crew duty day, or justified it's fatigue conclusion.  Even Chairman Sumwalt dissented with that conclusion:

However, Vice Chairman Christopher A. Hart and Board Member Robert L. Sumwalt III dissented on the inclusion of fatigue as a contributing factor, on the grounds that evidence was insufficient to support such a conclusion. Notably, the same kind of pilot errors and standard operating procedure violations had been found in other accidents where fatigue was not a factor.^{[3]: 171–173}

To state that fatigue was a contributing factor, and thus part of the probable cause, would be inconsistent with the above finding and would, therefore, disrupt this flow of logic. I did not feel, therefore – nor did the board's majority – that we had sufficient information or evidence to conclude that fatigue should be part of the probable cause of this accident.^{[3]: 171–173}