Because what happened at Kitty Hawk was (deep breath) the world's first sustained flight by a piloted heavier-than-air vehicle that achieved lift by means of its own self-contained internal system of propulsion. I think I got all the qualifications. Point is, while none of the individual aspects of what the Wright Brothers accomplished was by itself revolutionary, the fact that they were able to put them all together for the very first time was extremely significant indeed. The entire history of aviation can be — and has been, obviously — divided into what came before Kitty Hawk and what came after.