The idea was the first of its kind. It caught attention for its originality. It got traffic, the more traffic it got, the more attention it received from advertisers.
You can't create a new one and expect the same (or any) success, just as you cannot make a million dollars off of a new iFart like application.
Sometimes being there first and being original works. I am also pretty sure they knew the right people to show this to when they just started out so that it becomes viral...
Don't forget about psychology. The feeling of being part of something bigger plays a role. Getting people together on a network just to hang out is becoming passe. Getting people together to contribute to something bigger than themselves, even if it is just a mosaic of internet ads/photos, is interesting, the same way a Rube Goldberg machine is interesting. The sense of continuity and togetherness is in fact a component of viral marketing.