I think the problem is that "conspiracy theories" are defined as things that work with anomalies and somewhat outlandish induction rather than positive evidence that can be falsified or even verified empirically. Also, I think by this point 9/11 deniers have done so much lying, fudging of the facts, quoting out of context, and other assorted asshattery that they're simply not even trying to be taken seriously. They've become a band of circled wagons, assuring each other that they hold "the secret truth" that they alone are privy to, and that anyone who dares raise a dissenting question is one of those billions of evil, treacherous, curious outsiders.