Why won't the government classify it as such?
On November 5, 2009, Nidal Hasan entered the Soldier Readiness Processing Center in Fort Hood, Texas. After sitting at an empty table for a few moments with his head bowed, he jumped to his feet and shouted "Allahu Akbar!" before he began shooting. He killed 13 people and injured scores of others.
As time has passed since the tragic shooting, we have learned that Hasan gave a lecture to other Army medical staff about the Koran and stated that non-believers would be condemned to hell, have their heads cut off, and have burning oil poured down their throats.
It was also revealed that Hasan proclaimed himself as a "Soldier of Allah" on business cards and contacted numerous jihadi websites. Additionally, Hasan sought guidance from Anwar Al Alwaki as to whether it would be permissible under Islamic law to murder American soldiers.
During a hearing in court, he has stated that his motive in the shooting was to defend the "leadership of the Islamic Emirate of Afghanistan, the Taliban."
On the eve of the first day of his trial, Hasan provided 7 pages of documents to Fox News in which he states:
I, Nidal Malik Hasan, am compelled to renounce any oaths of allegiances that require me to support/defend (any - sic) man made constitution (like the constitution of the United States) over the commandments mandated in Islam ... I therefore formally renounce my oath of office ... this includes my oath of U.S. citizenship."
In the same documents, Hasan also details his contacts with Al Alwaki and praises him as a martyr.
Yet, despite this overwhelming evidence, the Department of Defense has classified the Fort Hood shooting as an act of "workplace violence," claiming that there is not sufficient evidence to prove that the shooting was an act of terrorism. The classification of the shooting as "workplace violence" has resulted in survivors being denied certain combat-related medical benefits.
[ETA: The Senate Report has more details regarding Hasan's radicalization and the missed red flags leading up to the attack.]