The problem is the way we watch TV has changed and with the internet young people don't tend to sit in front of a TV to watch their fav shows anymore. This is especially true with shows like TSCC and DH that are put on on a friday when young people the show is aimed at will be out. The Nielson rating system is really out of date and as such good shows are getting canned before their time.
Also reality TV doesn't help either.
1st part has almost nothing to do with it, ratings are ratings and they work fine, things only get moved to friday night when everyones out then they aren't fighting for viewers, already dying shows get put there. That Sarah Conner got a second season was surprising tbh.
Smallville, finally is on a friday meaning it shouldn't get another season.
But the reason so many shows are canceled is the second part, reality tv. No actors, so no fee's, someone like a Chandler or Josh from the west wing in Studio 60, will be getting 200-500k an episode(1st season 200k maybe, if it got a 5th season 500k-1m) reality tv just doesn't have stars. Even the presenters will be useless people not making very much at all. Writers could be on anything from 10-100k a show, and a big show will have anything from 5-30 writers. Again reality tv costs nothing, the producer and 3 interns coming up with dumb stuff for them to do isn't writing and costs nothing.
Reality tv is almost 100% pure profit, proper TV shows are mostly just trying to break even and if lucky, after 3-4 years can make decent money in syndication and dvd sales
Its a shame, Studio 60 for example, was superb, but money was short, thats why the two main guys weren't in for best part of 2 episodes, to save on costs. It was probably the best writen thing around in a long time, but too many people prefer watching reality tv and more importantly, other countries have to pay a lot more to show a Studio 60, than The bigger loser, or whatever other cheap and crap reality tv, so syndication is hard on the big shows now as other countries don't want to pay given the choice now.
Saying all that, Reaper was miles away from being one of the great shows around to be canceled, honestly surprised it finished the first season at all.
Sarah Conner had the same problem, it was being writen as a 10 season epic with plenty of time for loads of episodes about upset girlfriends, teenage angst, should I/Shouldn't I be in school. It was turning into a half action series, half teenage angst series, and 98% of people tuning into a Terminator series didn't give a crap about his angst. Action costs more though and CGI pushes costs up hugely, what they really needed was to write in a group of humans helping the terminators who wanted to help end the planet and start again. Introducing a cheap as chips way to add action without CGI and robots and continue the story.
. But there is a very good chance there will be a continuation of the show, albeit in the form of a 2 hour straight to DVD movie.
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