I found the thematic idea interesting, that the suffering of a tragedy is much more a factor for the world left behind than for the ones in the tragedy itself. I thought the story had an intriguing twist on a way to present its premise. (Instead of people learning to let go of the past, the past learns to let go of itself.)
I don't really know why it had to be 9/11. I get that it was inspired by thinking about that event, but I suspect that the hot-button nature of that topic will (and, indeed, already has) overshadow(ed) any discussion of the story qua the story. Every comment thus far has had to say something about the story "using" 9/11, and basically the well is pretty thoroughly poisoned at this juncture. Within the story itself, the need to give "due respect" to the idea of 9/11 meant that the first part of the story was a total drag. For me, it was just a series of, "yes, yes, heard it before, ten years of maudlin television coverage already, for pity's sake ten times more people die in more horrible ways every day, can we get to something else please?"
The story became interesting when the protagonist met up with the fading remnants of previous tragedies. To me, this was really the thematic core and the "important" bit of the story, the part that was trying to get itself told. I think the story may have worked much more smoothly if the protagonist had been the remnant of a non-specific tragedy on a smaller scale. Could still have been in a burning building and jumped and all that, even, and then met these centuries old (and thus safely neutered) tragedy-ghosts and followed more or less the same path to closure. Heck, a big enough small tragedy likely would have a little memorial to it, as well, like those crosses and wreaths people set up at the sites of car accidents. That would have sidestepped the need to belabor the point of 9/11 both in the story and in the comments thread.
I have no idea why everyone is so het up about the idea of victims of a tragedy continuing to suffer because the universe is unjust. Guys, it's a spec-fic story: the author isn't hoping that it was true. Getting angry or upset by that just confuses the heck out of me. Not liking it, sure, go nuts, no one says you have to enjoy it, and frankly I'd be a little freaked out if someone liked the idea that innocent victims got to keep on suffering for no reason, but treating it as though the positing of a what-if is the same as wishing for it just seems weird to me. Some douchebag posts on Facebook about how the tsunami/earthquake in Japan was "revenge for Pearl Harbor," yeah, tell that person how upset you are with them, but this is an author writing a story that basically just says, "Hey, what if the universe wasn't actually fair and you had to make your own meaning in it even in the face of tragedy?" Frankly, I haven't seen a whole lot of evidence that the universe IS just or fair, so I'm not seeing where this is an outre and offensive position.