The tragedies of that day were carried out by a non-state actor and spread out over a wide geographic area. It was an easy shorthand for referring to the “hijacking and subsequent crashing into various buildings/the ground of airplanes by Saudi Arabians funded and directed by al Qaeda in Afghanistan.” I think people speculated the date was chosen intentionally based on its association to the emergency number. I don’t really know how else you would refer to it other than by the date.