Because Schneier in particular has shown numerous weaknesses in airport security systems. And what you've described as "disdain" amounts to treating airport security like any other kind of security system, evaluating it critically, and correctly realizing that it has almost zero value, a huge cost (in both money and time), and a huge impact on civil liberties.
In other words, he has very effective criticisms, which makes him unsuitable for hearings designed to publically demonstrate a lack of effective criticisms.
This comment basically presumes that I disagree with Schneier and that the way to rebut my point is to re-argue what Schneier has argued. But I agree with Schneier, as do I think most people --- particularly security-minded people --- who pay attention to the TSA. My point is that Schneier has a lot of credibility with nerds and a lot of credibility with Jeffrey Goldberg but perhaps not as much credibility elsewhere as, say, an FBI agent who also disdains the TSA.
Not trying to rebut your point, just answer the question you posed. I don't think anyone has assumed that Schneier represents the most effective critic, just someone with reasoned and thoroughly correct arguments, and background in a relevant field. I agree that Schneier's voice may not carry the most weight, but that just suggests the need for more voices.
In other words, he has very effective criticisms, which makes him unsuitable for hearings designed to publically demonstrate a lack of effective criticisms.