Whether the goverment has something to hide or not, any information regarding 9/11 should be declassified. The American public should not be kept in the dark about such a major incident. Even looking at this with all conspiracy theories aside, it would aid in giving closure to the families of those lost.
The American people deserve to know what really happened on 9/11. It has been 12 years now, Osama bin Laden is dead, so what's the harm? Does the government have something to hide? Declassify the documents and let a new independent investigation take place. I have a feeling it would be the start of a new American Revolution.
Classified documents become declassified at longer intervals for a reason- to ensure that no vestige of the original reason for classification remains.
The default for lower-level documents is 10 years. Higher level classifications, like the 9-11 documents, get reviewed periodically every couple of years after that until 25 years, where the review process is automatic and only nine exceptions exist to keep the document classified. At 50 years there are only two exceptions, and after 75 years documents are automatically declassified unless special permission is received.
The National Archives holds the office for document declassification reviews, and also responds to any Freedom of Information Act queries. In the case of the 9-11 documents, classification has remained for most documents (or for the few released there were heavy redactions) because Al Qaida is still active.