Why do so many tv shows get cancelled?

Throughout the summer I've had the opportunity to go back and catch up on tv programmes that I somehow lost touch with, as well as start watching a few series that I over looked when they first came out.
Due to these programmes having been out for a while now, I was lucky to find that there were plenty of episodes already out, allowing me to have a few tv programme marathons.

Whilst going these marathons, it was at this time that I noticed how many of these tv series and other programmes I liked in the past, either got cancelled and cut short, a few of these being; Stargate Universe, Lie to Me and The Dresden Files.
The Dresden Files came out about ten years ago or so, and I watched it on day one, waited every week for the new episode to come out, and then the season ended and that was it. The season that aired consisted of about 11 episodes, however I always thought that there were still more episode left but they just wanted to pull the plug early on it and so cut it short.

The programme Lie to Me, is one that I just recently discovered and was hooked on to after watching just one episode. What's quite sad though is that I looked to see if it was still on tv and on what season. What I discovered however is that it got to its third season and then wasn't renewed for a following season. Why this is quite sad is because with every episode I watch now I'll now that I'm one episode closer to it ending and there being no new ones left.

Some programmes are lucky in the fact that the no somewhat ahead of time that they won't get renewed and so can tie up story lines and reveal the plot lines in full before it ends, however with some programmes, such as Stargate Universe, seemed to have less time to react, and had to make the best ending out of what was available, however this meant that not everything was answered, and things were left unfinished.

Sometimes I just wish that all of these networks cancelling shows would forget about the money side of things, and would just continue to make the shows because they have fans and an audience out there who want to watch them. Hopefully there'll be another long lasting tv show soon to get involved in, and we won't have to worry about it disappearing after the first season.
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