Why? Because the leaders of the country and many of the people so STRONGLY want things to be the way 'they used to be', they cannot embrace change or consider the unthinkable. It is almost as if they fiercely WANT it to NOT be terrorism, but that has no effect upon the objective actions of a terrorist cell committed to our destruction.
It is a good case study in human discomfort with 'Change' and propensity for 'Denial'.
In fact, "The United States of Denial" might be a more fitting title now.