I think there are arguments that Reunion is not doing as horribly as people think. It's ratings are low, but until this week (after it was announced the mystery would not be resolved), its ratings were comparable to or better than just about every other show Fox has aired on Thursday night at 9 pm. And that was with a weaker lead-in from The OC than Fox's past Thursday night shows have had. Reunion also has a pretty rabid fanbase, and I suspect there are a significant number of people recording the show that are not reflected in the ratings.
The fact is that Fox has nothing that will do well in that timeslot. The comedies that will replace Reunion in February will be slaughtered against CSI and the Olympics. I think Fox should have bit the bullet and given this one to the fans. It won't do better with any other shows, but it could have made them believe it is worth getting invested in future Fox shows. I know that I will not be watching any freshman dramas on Fox next year. If a show survives the full season, I can rent the DVD from Netflix and then make a decision about season two.
Also, I disagree with the person who said Fox promoted Reunion. While Fox promoted Reunion over the summer, it pulled nearly all of its advertising after the presidential address bumped the second episode. Other than a few promos during shows like
Wife Swap or during The OC, there were almost no Reunion promos. It was almost never mentioned during the baseball playoffs or during football games, which are Fox's biggest promotional opportunities. There would have been ways to promote the show, using the weekly "twists" to drive interest, but Fox just didn't bother. I think Fox wrote the show off early, but just waited as long as possible to make the announcement.