They are media specialist!

Let's begin on the terrorist side. Terrorists are media specialists. They just know how to get your attention and guarantee media coverage by enhancing their actions' news value. How?

(1) the degree of violence applied; the number of the status of the victims

(2) Use of symbolism : actions are carried out at a time of place with a special meaning for the attacked country

(3) performance of a very spectacular or special deeds likely to appeal to the emdia

The 911 attack possesses all the above elements and has been said to be an ultimate PR stunt.

Brigitte Nacos, professor of political science at Columbia University, commented ,"it was the very terrorists who loathed America's pop culture as decadent and poisonous to their own beliefs and ways of life turned Hollywood-like horror fantasies into real life hell."

Further reading:
Hobsbawm, J. (2001, September 16). On public relations: How they held all of us hostage. The Observer.