Does anybody know?

From Wikipedia (no exact link):

The FBI stated that evidence linking Al-Qaeda and bin Laden to the attacks of September 11 is clear and irrefutable.^[63] The Government of the United Kingdom reached the same conclusion, regarding Al Qaeda and Osama bin Laden's culpability for the September 11, 2001 attacks.^[64] But so far, the U.S. Justice Department has not sought formal criminal charges against bin Laden for the 9/11 attacks. This has provided what some call "fodder for conspiracy theorists who think the U.S. government or another power was behind the Sept. 11 hijackings." ^[65]

Is it lack of evidence?

Considering the old maxim - a prosecutor can get a Grand Jury to indict a ham sandwich - what is the reasoning to NOT indict Bin Laden?

Any thoughts?