Questioning a theory requires an alternative theory
There is an idea that, in order to challenge an existing theory, you must have an alternative theory which explains the same events or data.
This is not, in fact, the way science works. Darwin did not have to discover continental drift before publishing his observations of fossil jungles in the Antarctic. Astronomers did not have to discover the theory of relativity before noting that Mercury's observed orbit was slightly off from what the physics of the time said was possible.
The observation of a discrepancy between theory and observed fact is often the first step towards scientific discovery; attempts to suppress observations just because they do not fit with an existing theory are therefore anti-scientific.
Since the public lacks access to most of the significant bodies of relevant evidence, much less the power to question uncooperative witnesses under oath, there is absolutely no way to build a solid case for any alternative theory without some kind of official re-investigation.
Thus calls for a new investigation are countered by appeal to absence of the evidence which such an investigation would provide -- and the official theory is allowed to stand despite being deeply flawed and substantially erroneous.
Beyond that, this claim is also used to argue that "truthers" should speculate on possible alternative theories if they want to gain credibility. This speculation is then used in straw man attacks claiming that it is the main premise of the truth movement – opening the door to enumeration of all the errors in logic that would have been the case were this actually true, stated as actual errors committed by truth advocates.
The combination of these two arguments, plus lack of access to the necessary evidence, basically places "truthers" in a double bind: they lose the argument if they don't speculate, and they lose it if they do.
- This claim is included in The 9/11 Position Quiz, question B2.