Outliers be outlyin, yo.
We sensationalize terrorist attacks specifically because they are so uncommon. It's part of our classification system, which has done a fantastic job of keeping us alive, as a species, for so long.
When we see something new or uncommon we spend a great deal of time paying attention to it, figuring it out, understanding how it impacts us (individually/selfishly) and once we've classified a thing, we cease to pay much attention to it or new instances of it - only enough to see if it matches well enough with a previously seen thing that we can apply the proper response and move on.
It's not even a form of fatigue as some seek to describe it, it is, as @drawn-inward correctly points out, a form of desensitization. We are no longer sensitive to things we've previously classified. As we raise our children, they first look at us to see what our reaction is to a new thing. Go ahead and go to the park with toddlers and newborns and watch them - see how often they glance around, then glance at their parents to learn the proper response to a new stimulus. They don't always accept that, but they seek clues and hints and learn, in this way, lessons we've already learned but don't think to specifically pass on, teach, or train. It's below our personal radar. As we age we look to other "authorities", some the news, some certain pundits, some look to mentors.
And the terrorists have to keep changing their attacks up, because the same old bombings are not bringing the attention they seek (as well as it simply being harder to conduct a similar operation once the authorities figure out the means and method used for the first one).
@Dave, it appears, is suggesting that we actively desensitize ourselves, as a society, so that terrorist attacks don't become shocking major new items every time, and we become only as interested as necessary to close off one more loophole/means/method. I don't think it's possible, but I don't see a problem with the suggestion. It simply goes against human nature and our instinctive drive to understand new stimulus well enough that we can classify it and "program" our response.