Not trying to sound condescending here, but are you old enough to remember it? At the time it was an earth shattering event. I'm not old enough to remember the JFK assassination, but older people say that's the only thing comparable.
It wasn't the number of people who died as such. Like you say, more people have died before and since of all kinds of natural and man made disasters. For me, with 9/11, the shocking thing was seeing the twin towers which were icons not just of New York but America, the great world power, come crashing down. It was like something out of a blockbuster movie, something you would never expect to happen in real life.