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The show didn't last for a variety of reasons.
First and Foremost, it didn't get high ratings. That could be due to it's bad time slot. (Firefly, John Doe, and a 3rd show I think) were in the Friday Night slot. Most shows on Friday Nights don't do well since that is a typical US 'Go out and Party' night.
Some have said that it was advertised poorly. I wouldn't know, I saw advertisements and watched the show from beginning to end based on seeing the commercials.
Some have said that because the Pilot wasn't aired first. The first episode shown was 'The Train Job' which drops you in the middle of the action, but doesn't do much for back story, so you had no idea who these people were and what was really going on. Strangely the Pilot episode was the last aired episode...
Also, as much as I love the show, I can understand where many people don't think the idea of Cowboys in Space is appealing.
I could go on, but this is probably more than enough for now. I think we should be happy that the DVD release had the un-aired episodes, and we should be greatful that we got Serenity. For all the bitching that people did to get that show back, the fact we got a movie was amazing.
The more amazing thing is that the people who whined and complained couldn't be bothered to see the movie a couple of times in the theatre and drag their friend so it could actually be a commercial success. I can't tell you how pissed I was at a group of friends who downloaded the movie. I saw it on opening night in a packed theater, then again the next weekend in a 1/4 filled theatre...