Labor politics.
Y'see, it's little known, but Video Game characters have a union, at least the major ones do (the Goombas have been trying to unionize for years but still haven't).
Now, while Video Game characters are, by nature, entertainers, they do have limits. So it's not like they can show up in a new game every week.
Well, the main problem arose from the Mario franchise players. They have a busy month or so ahead of them, not to mention a few are still bushed from the Summer releases.
Think about how much they are scheduled to do over November. First they have to compete in the Olympics freaking games against the Sonic characters (and as everyone knows, doing all those events in the Olympics must be hard, even for people like Mario and Sonic). Then Mario and gang has to do Super Mario Galaxy, which involves heavy travel, many stunts and other risks. And then they have to throw a Party on the DS, and as anyone who has ever been to any of Mario's parties knows, those leave you downright tired. Needless to say, by December 3rd, the Mario gang would have been working well above the union limit of games.
Link would also have been tired, he still hasn't fully recovered from doing Twilight Princess on 2 different consoles, and then doing Phantom Hourglass, and still having to do both a crossbow game before SSBB came out.
And the pokemon? You try doing both gamework and a TV show at the same time! It's a miracle they show up at all!
While they had bent these rules before, that was primarily to make cameos or appear in remakes. So Union representative Phoenix Wright (doubt you knew he also had a labor law degree as well, huh?) yelled OBJECTION! and told Sakurai that he had to delay the game.
Stupid lawyers.