The short answer: Cover-up.
As is routine practice following every conflict, the Army asked the RAND Corporation to prepare a study of the planning and post-war Iraq so the Army can learn from experience for future conflicts. In the summer of 2005, after 18 months of study, "Rebuilding Iraq" was done. Again, as is routine practice, RAND submitted a classified version and an unclassified version "hoping that its publication would contribute to the public debate on how to prepare for future conflicts."
As would be expected, the report was critical of all the players in the disaster we now call Iraq including the White House, DoD and State. It is against Army regulation to prevent an unclassified version of such studies from being released to avoid criticism, and naturally, that's exactly what the Army did.
And just as naturally, the New York Times got a copy anyway.