Because if we all wailed and sobbed over thr daily individual events that caused high aggregate casualty numbers such as road deaths, fags, Jeremy Kyle viewing related suicides etc, we'd all be suffering PTSD.
We're intellectually conditioned to cope or handle the world in a certain shape and condition, and death as a result of lawful activity such as driving or sports, other accident, illness, even murder is sufficiently commonplace that our psyches have adapted since birth to manage that. An act of sheer terrorism undoubtedly intended to cause mass death, gross injury etc, is vastly out of societal norms and quite worthy of both condemnation and expressions of sadness and sympathy. I would suggest that someone who is isn't at least intellectually, if not emotionally, concerned or moved by such an event might be a sociopath.