Wow, I managed to communicate something to a Twoofer.
The reason why he hasn't been indicted of course, is that in order for this to happen, it's not enough merely to prove that (a) al Qaeda is responsible for 9/11 (b) OBL is head of all Qaeda: one must also establish that he gave the orders.
Until then, he can't be indicted.
It's the same reason it's so hard to indict Mafia bosses.
Of course, in the fantasy world in your head there'll be a totally different reason why he hasn't been charged, involving the complete innocence of the filthy murderers who perpetrated 9/11 and the complete non-involvement of al Qaeda in the bloody crimes that they committed, but that's the actual reason.