Smash Run being local only, and the fact Gema had 1,400 street passes - the maximum possible - says to me things are just different in Japan. People are in closer proximity, population is denser, and there is no reason for online.
Whereas in America it is basically unfeasible to play local multiplayer unless you already know a specific group of friends. I've had a 3DS since launch and take it with me everywhere - including the toll booth I work at that sees thousands of vacation goers a day pass through - and I have 200 Miis in my plaza.
I will never, unless I drive to somebody's house and sit down with them with the express interest in playing Smash Run, ever be able to play Smash Run with another person.