bin Ladin has always clearly admitted responsibility for previous attacks, why not 9/11?
bin Ladin has made many statements that clearly show responsibility for the 9/11 attacks, as far as we can see. Further, al Qaeda members have explained in an audio-taped interview how the attacks came about, and were planned (see). Some people want still more, but the past record shows this is how bin Ladin works. He rarely directly claims responsibility, probably 9/11 is as close as he's ever come to it. By way of an illustration, here are some comments from an interview in May 1998:
John Miller: Wali Khan Amin Shah was captured in Manila. American authorities believe he was working for you, funded by you, setting up training camps there and part of his plan was to plan out the assassination or the attempted assassination of President Clinton during his trip to Manila.
bin Ladin: Wali Khan is a Muslim young man; his nickname in Afghanistan was the Lion... He was a close friend and we used to fight from the same trenches in Afghanistan... As to what you said about him working for me, I have nothing to say. We are all together in this; we all work for Allah and our reward comes from him. As to what you said about the attempt to assassinate President Clinton, it is not surprising. What do you expect from people attacked by Clinton, whose sons and mothers have been killed by Clinton?...
John Miller: The federal government in the US. is still investigating their suspicions that you ordered and funded the attack on the US military in Al Khobar and Riyadh.
bin Ladin: We have roused the nation and the Muslim people and we have communicated to them the fatwahs of our learned scholars who the Saudi government has thrown in jail in order to please the American government for which they are agents. ... Among the young men who responded to our call are Khalid Al Said and Abdul Azeez Al... and Mahmud Al Hadi and Muslih Al Shamrani. We hope Allah receives them as holy martyrs... Yes, we have instigated and they have responded. We hope Allah grants their families solace.
Were these clear claims of responsibility? Not really, but it's not difficult to believe that bin Ladin knew more about both these cases. Further, his final claim is very similar to one bin Ladin used later, about 9/11: he's not "responsible", but he has incited others to take part in an attack:
"What many leaders have said so far is that America has an indication only, and not a tangible proof. They describe those brave guys who took the battle to the heart of America and destroyed its most famous economic and military landmarks.
They did this, as we understand it, and this is something we have agitated for before, as a matter of self-defense, in defense of our brothers and sons in Palestine, and to liberate our sacred religious sites/things. If inciting people to do that is terrorism, and if killing those who kill our sons is terrorism, then let history be witness that we are terrorists".