This is not the case. Many of the information disclosed after 9/11 show that the US intelligence agencies are aware of Osama bin Laden. It is only because the US intelligence agencies have complex systems, serious internal consumption, lack of unified command, cooperation and scheduling, etc. Take the possession of Osama bin Laden seriously. For example, the FBI agent John O'Neill was the person directly responsible for investigating Osama bin Laden before 9/11, but he eventually retired from the FBI and became a victim of a terrorist attack on September 11. It can be said that John O'Neill has an almost crazy obsession with investigating bin Laden. In order to investigate Osama bin Laden, John O'Neill needs a lot of intelligence support, but the Bureau of Investigation did not give enough support to O'Neill's work. The FBI has always been ruthless for employees who have a distinct personality, ambitious and dare to challenge mainstream thinking.

John O'Neill can be said to be the closest person to Osama bin Laden before the 9/11, but he was ruined by the FBI and other intelligence agencies' endless internal consumption. In 2004, the investigation report issued by the Independent Investigation Committee of the 9/11 Congress showed The CIA and the National Security Bureau have long mastered the information that John O'Neill needs, but refused to share it. This mechanism for refusing to share information among government agencies comes from the Federal Criminal Procedure Regulations, which prohibits the disclosure of any material related to criminal investigations. Later, this rule was restricted by the FBI to limit the investigators of this Council. Means: It is strictly forbidden for anyone to share information, even for agents in this Council. Like the Bureau of Investigation, the CIA has also turned this barrier between its own and the Bureau of Investigation into a system. The CIA believes that sharing intelligence may undermine “sensitive sources and means”. The National Security Agency directly limits the transmission of important information. Agents of other agencies can only see brief reports on intelligence, and cannot obtain the original monitoring records of the National Security Bureau.

From these details, we can see that the main part is that the United State didn't make a very comprehensive protection.
Comprehensive protection