Some shows are cancelled for reasons that are not well reflected by ratings.
For example, Commander and Chief (a show I was not particularly fond of by the way, so it serves as a good objective example of this) was cancelled largely for things that had nothing to do with what was actually on TV. The show had internal fights behind the scenes, and lots of people being fired, rehired, and fired again. It had massive gaps in scheduling, with the show being sheduled to air one week and then being put off for another three weeks to "retool" with the new crew. It switched days around, and even got misadvertised.
So, it's no surprise at all that it is now cancelled. But, though technically it was about ratings, realistically it was about the show being tanked by things that happened behind the scenes.
In my opinion, similar things happened with the shows Firefly and Wonderfalls. Both had executives shifting the schedule around and delaying shows in a way that killed the ratings.
Some day, when all people have a DVR like Tivo with the ability to record multiple shows at once, executives will not kill a show as easily with their scheduling silliness. For example, I watch the show Twins (not great but okay), and it is cancelled and they are leaking out remaining episodes on an almost entirely random basis right now. But my Tivo still picks it up when it shows. If I didn't have a Tivo, there is pretty much no chance I would catch the remaining episodes.
Tivo frees you from the slavery of your television schedule. Buy Tivo. I command you to buy Tivo! (this message brought to you by the stockholders of Tivo
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