IMO there isn't just one "secret weapon" as to why the Wii is successful. There are several of them, including their good marketing, innovation of motion controls, family-friendly theme, classic gaming icons ( Mario/Zelda/Metroid), Simple/Fun games ( Wii Sports and other party titles ) and at the time the system being much more cheaper and affordable than it's competitors. It's just my opinion though, so all of this could be wrong.
"The Common Cold Isn't So Common During The Cold"