Because the Crusades are often understood within a larger framework that says that Islam is the gentle faith and Christianity the violent one. Karen Armstrong would have us believe that Muhammad was a pacifist. Take Major Nidal Hassan, the man responsible for the Fort Hood massacre. Had an evangelical Christian of the nutty sort gotten up in front of Army psychiatrists and talked about how much he respected people who shot abortionists, he would have been out of the Army an hour later. But everybody tiptoes around the issue of Islam.
Several months after 9/11, former President Clinton gave a speech at Georgetown University in which he apologized for the Crusades. He said we had much to be sorry about, and we bore some of the guilt for sending those airplanes plunging into the Twin Towers. Now, Clinton isn't a nut. He's not an anti-American. He's just been miseducated. He's been told a whole lot of nonsense about the Crusades.
The interview itself is from Patheos.