Because terror is an unknown that causes fear. People are comfortable thinking they know all the answers.
Gun death - very little unknown.
Bad guys: use it as leverage to commit robbery, rape and murder.
Good guys: use it for self defense to kill an intruder committing robbery, rape and murder.
People don't care too much about war death, traffic DUI drinking-and-driving death, or air pollution cancer death, or "infected at the hospital" death, airplane/bus/train crash death.
25 years ago, the popular trend was worrying about AIDS death, and feeding the African children dying from hunger.
Now that is also "meh" apathy.
All those death categories are in the thousands every year, and if they get reported on the news its less than 5 secs mention on the news "...car accident fatality on the freeway in Los Angeles, car bomb in Iraq kills 3, and now with the results of the Oscars movie awards..."
Terror death is a tiny number, but aside from politics, it's a factor that we believe is out of our control, and creates fear because we don't know how to mitigate it.
however we believe that if we have airbags and car insurance, we can survive a car crash, so that risk is mitigated, and we don't worry too much about it, other than saying "drive safely, good bye"