Because publically Bush said on more than one ocassion that any demands for evidence were not acceptable. Have you been paying attention? There is no credible evidence in the public domain that bin Laden is actually responsible.
Afghanistan did say they would turn over bin Laden if the U.S. would show them the evidence. We refused. We said that such a demand was yet more proof that they were uncooperative.
Blair released a white paper that detailed the evidence. The Taliban dismissed it offhand. That is not cooperative.