Every U.S. president since Carter is responsible for not having dealt more decisively with the Islamic terrorist threat.
When Carter handled the Iranian hostage crisis with such weakness, terrrorists realized you could take on the mighty American superpower and win. Reagan, for all his strong foreign policy and success at fighting Communism, should have responded to the bombings of the U.S. Embassy and marine barracks in Beirut with similar strength. But he didn't -- he pulled all troops out of Lebanon the next year.
Could Bush Sr. have done more? Absolutely. Just about all American citizens and political leaders could have done more to fight the terrorist threat. As a nation we clearly did not do enough between 1979 and 2001 to disrupt and prevent terrorist activities.
Bill Clinton is especially vulnerable to this criticism because there were several al-Queda attacks during his administration, including embassy bombings and the bombing of the U.S.S. Cole., and he didn'd do much in response. Al-Queda was a growing threat that was allowed to continue to grow.
Clinton also suffers from a perception that he was too distracted by his "personal issues" and his poll-driven style of politics. What the country needed in the 90s, to deal with terrorism effectively, was a president who could take a bold stand, rally the American people, and take (probably unpopular) decisive action to stop them. A person, in other words, who would do what's right for the country, even if it was politically unpopular. Bill Clinton clearly was not that man.
And say what you want about GW Bush, he certainly has his flaws, but I think that most people would agree that 1. he is decisive; and 2. he is doing what he thinks is best for the country, even if it is politically unpopular.