Unfortunately, America runs on "national interest". So, those whose evidence would be decisive are keeping quiet. Bush, Cheney, Powell, Rice, Hillary & Bill, Biden, Lieberman, McCain, Kerry, Obama, most senators and congressmen and the top brass know the truth.
When national interest demands it, all documents about 9/11 will be declassified.
Another factor is the political culture of assassination of the dissenting opponents. Anyone among the above seen ready to open his mouth will be silenced by the CIA.