Well, for me personally, the issue is I hear about this kind of thing EVERY DAY, and granted it isnt tragic on the scale of WTC's event, but the fact is that people are dying every day because of radical political activists.
Sure, just not people in relatively stable countries that someone here is likely to know personally. You expect people in unstable or war-torn countries to become collateral damage. You don't expect one of the largest structures in the country to be destroyed in one fell swoop in any situation short of World War III.
The only reason people here are horrified is because it happened here.
Also because it was a large-scale, unexpected attack on a large group of civilians in a place they felt safe. For a short period of time it was pretty scary for those watching it happen that were old enough to extrapolate the possible implications for the rest of the country. For a lot of people it shattered their sense of security. That might not be a bad thing long-term, but it tends to leave a mark psychologically.
I can understand that it might be easy to dismiss for people that were too young to understand what was happening at the time. For others there was a huge degree of uncertainty attached to it that most of us weren't accustomed to. "Emotionally jarring" would probably be an understatement.