How do video games get sold out?

I wanted to buy New Super Mario Bro. Wii for my wife.   I wanted to wait until after Christmas to see if I could get it cheaper.   So a few days before Christmas I went into my local Wal-Mart to see if they had any of the game in stock, come to find out they were sold out. Although it is not the best game on the Wii, it is very popular because of Mario. Anyways that being said, Nintendo had to have known that it would be a big seller this holiday season, so why are places sold out? How hard is it to manufacture these games? Especially ones that everyone knows will be popular