I blame WiiFit!
But, more seriously, PS3 and XB360 rely more on hardcore gamers, that buy more games, and also on more games that have a single player experience with a beginning and a end, once finished, the player may continue on multiplayer, if the game has an appealing one, or buy another good enough single player game, this is quite sure to increase tie ratio.
OTOH Wii has WiiSports from the start and a lot of other games with massive replayability and party gaming value, you can defy yourself or your friends more time without "exhausting" a game.
I can say this for my personal experience with Intellivision, in more than 15 years I used it more or less intensely, I bought a lot less games than for the PC in 12 years (before 1997 I hadn't a PC at home, I played PC games at the University): my PC games are almost all mainly single player, but for the Intellivision I had some single player and enough dual and/or single player ones that lasted me really a lot.