I notice all the price drops for Wii games from gamestop as well. I go there and also the movie trading company because they have a lot of cheap games for multiple systems.
I think they sell so cheap because the games no longer sell for the Wii like they used to, and gamestop probably has made so much profit off these titles, that they can just sell them for little to nothing. That's good for the consumers because someone like me who likes to collect games will take advantage of those deals.
The most Wii games I bought at once was 23, and I didn't even spend $100 dollars at gamestop that day.