I have no idea why it hasn't been released. Lots of information has not been released. There are lots of questions and few answers at this point.
Have you seen this letter from Rep. Michael T McCaul about the stonewalling?
http://lang.dailybulletin.com/projects/pdfs/012007_trialprobe.pdf
January 19, 2007—Congressman Ted Poe requests previously unreleased information on the case from the Department of Homeland Security’s Office of Inspector General (OIG) under the Freedom of Information Act after the OIG reneged on a 2006 agreement to provide an investigative report to Rep. Poe and other members of the Texas Congressional Delegation.