If it had happened anywhere else it wouldn't have had such an impact. It wasn't just horrific for those directly affected either it was horrific for everyone on the planet. Even those perpetrating it. It spiraled onto everything.
It was a pivotal moment and like the assassination of Arch Duke Ferdinand was the 'crime of the century' and will have a knock on effect for a long time to come (altho the death of ADF is still contributing to it all as well).
It isn't just the deaths of those people. It is the perfect combination of factors which still (and will forever) make it one of the most astonishing, awesome, audacious, mind blowing acts of human will ever perpetrated.
Natural disasters are terrible, but this was an act by people against people. Like the holocaust it transcends comprehension of the will and hatred needed to achieve.
They location was iconic, and a perfect place for this kind of message to strike - it symbolised western decadence, greed, hubris even, was populated by people from many countries but still was on US soil. Then to use the US planes which are another symbol of the above as weapons against themselves.
It was when the west realised, really really, realised there was whole chunk of the planet who had diverged totally from us. it wouldn't have shocked me more if it had been an attack from another planet.
Innocence died and we had to wake up and see the world as it really was.