Smartphones and tablets are not good for games when you visit your parents for Christmas. You can't play Angry Birds with your kids on your phones. You can't get together with a group of friends and laugh your asses off while playing Smash Bros. or Mario Kart on your phones.
A great point, one great advantage the Wii uses is the real life interaction that its games bring to the players, in contrast to Xbox and PS games which no longer support local multiplayer. It moves against the flow of social interaction, something society in general is in desperate need of saving.