Regarding Game Party and Deca Sports.
First of all, since I own all these games released in North America, I feel that I can speak to the quality of them. (No one else who commented on them lists these games, so if they own them, they do not admit it).
Deca Sports was hyped as being the unofficial "Wii Sports 2" by some publications. However, I should have suspected something was up when Nintendo would not let Hudson use Miis in the game (even though Hudson has done a lot of second-party development for Nintendo). The game is only team-based, meaning you play as a pre-made person you select on one of eight teams. The events range from excellent (badminton) to frustrating (kart racing). Too many are closer to frustrating than excellent, however. It originally sold for $30 but has since dropped to $20.
Game Party came out to little fanfare or expectations. It had a few indoor games of generally acceptable quality. They were fun to play in short bursts. There were flaws (the cup pong physics were not great. The order was always fixed -- meaning the computer always went last, the trivia was lame). But it sold so well that Gamestop could actually move it for $5 over the $20 MSRP.
Game Party 2 included all but the air hockey from the first game. But it did not read the old records. And it made some games (such as basketball hoops shooting) too easy. Meanwhile, some of the new games (such as lawn darts) had broken physics. It was not worth the $10 increase in price to $30. It also has sold roughly a third as many copies as the original (still a healthy 700k in WW LTD sales).
What does all this mean?
It means that those who bought Deca Sports (Deca Sporta) and felt like the game did not live up to expectations probably would not buy Deca Sports 2 (Deca Sporta 2). And people who have seen half-hearted sequels are going to be much more weary. So expecting sales to be as strong as the original were folly.
It is interesting to note that even after that first-day performance, Deca Sporta 2 was still the 10th best selling game in Japan last week, third best-selling console game, and second-best selling Wii game (if I am reading the chart in the OP correctly).
Mike from Morgantown