A big factor is that there is significant demand in all 3 regions while it was a near simultanious launch.
+1 "core killer" Zelda Twilight Princess
+1 "non-core killer" Wii Sports.
+1 insane amount of positive press the system has recieved
+1 The Wii remote makes it easier for non-gamers to pick-up and play compared to traditional gamepads
+1 Very small, compact and stylish look resembling the Ipod
+1 Virtual Console for former gamers to get back into games
+1 Price
+1 Extremely positive word-of-mouth among casual and non-gamers, which is completely new for them for a launch system
Nobody is crazy enough to accuse me of being sane.