Why do we respond so viscerally to a terror attack that kills say ... 20 but not to 30,000 killed by cars p/yr. Is it the attack itself?

Possibly because terrorism is a violent and war like intrusion into peoples lives whereas driving deaths are a preventable action that most people don't want to confront since we all drive and could potentially kill someone while doing so. Just my humble view.

MSG Davis, Sir; probably for the same reasons we neglect our Veterans, but cater to our felons..

Numbness.

The human capacity to get used to anything is scary. We are the most adaptive creature to ever live on planet Earth. So much so that we have adapted to ideas like Preventable Heart Disease, Car Accidents, and Cancer being commonplace... yet "statistically irrelevant" events are what make us angry, because we are so Arrogant that we believe we can control those...