I think "incompetent" is not the correct word to describe the situation. Try "complacent". The Cold War was over and we had won! There was not another nation on the planet that could best the U.S. on the conventional field of battle. All other foreign terrorist attacks had taken place "over there" and not on U.S. soil. No administration up to that point had seen it as an emerging threat and therefore did not take it as seriously as it turned out to be.

Additional security measures also costs money. It is terribly hard to justify spending millions or billions on a problem that people do not think exists.

Regards,
Bean Counter