Have you forgotten?
Do you think 2015 will be the year when enough people demand a new, independent investigation... and finally bring the *real* perpetrators to justice?
Nicholas Levis, an original 9/11 skeptic and one of the founders of 911Truth.org, has stated:
Staging 9/11 as an inside job is going to work best (in fact, is likely to work only) if there actually exists an active network of anti-American terrorists who are deeply committed to killing Americans in response to U.S. policy. In other words, those who would blame Qaeda need a (relatively) real Qaeda. A partly-real enemy is much better than an entirely fabricated one.
The most robust way for insider masterminds to stage 9/11 and get away with it is to arrange for their agents to infiltrate among "real foreign terrorists." Let them come up with their own plots (or plant plots among them), choose a plot that will produce the results desired by the masterminds, and see that through to fruition. At some point, the masterminds and their agents will hijack the plot from the would-be hijackers, to make sure it happens. You won't risk the whole game on the ability of amateurs to get away with it, you will help them along or even replace them (with a remote control hijacking, for example). But it's best to have "real terrorists" in play. They leave a more solid trail of evidence internationally. Cops and agents and academics of two dozen countries can honestly confirm the existence of an al-Qaeda network. That way there is less need to initiate outside observers into the plot and you don't have to hope they are all stupid, as they would have to be to fall for a complete fabrication of "Qaeda." (Qaeda at this point is just a term of convenience for the Islamist extremist networks.)
The best result would be for a whole bunch of Islamist extremists running around believing that their crew pulled off 9/11 all by themselves (how inspiring for them!). The patsies should believe they actually did it. This was the case with the Reichstag Fire and Marinus van der Lubbe: the patsy believed he had done it.