At the 9/11 Commission hearings, little actual evidence was ever produced. Many individuals were not sworn in, critical witnesses were either not called to testify or were permitted to dictate the parameters of their own questioning, pertinent questions were omitted and there was little follow-up. Whistleblower testimony was suppressed or avoided all together. The National Security Agency, an intelligence agency that is responsible for the collection and analysis of foreign communications and foreign intelligence, was barely investigated at all.
With the narrative of the 9/11 Commission's final report predetermined and with the preexisting intention to never hold anyone accountable in place, the 9/11 Commission was doomed to fail as a real investigation. The end result of the 9/11 Commission's work was that some of the recommendations that they produced were in fact, based on distortions and omissions. Since their mandate of a complete accounting was ignored, the recommendations were incomplete at best.
There was clearly no desire on the part of Congress to force the Commission to meet its legislative mandate. Accordingly, there were no repercussions for the fact that the investigation and its recommendations were incomplete. It could be surmised that holding no one accountable was more important than uncovering and disclosing the truth.