Why Do the Good Shows Always Die?

I can think of dozens of shows that were cancelled before their time, Arrested Development, Carnivale, Studio 60 on the Sunset Strip and now the massively underrated Outcasts will join the ranks. I used to admire British shows and how they only ran for two or three seasons, they knew their time was up and instead of being cancelled, the actors just didn't want to do another series.  Also they only have 6 or 8 episodes, which makes people more keen to keep following the series. Classic examples are Spaced and Green Wing. Both ran for two seasons (exception on Green Wing special), both were hilarious, both have been given cult status, and rightly so, but both ended because the people involved decided it was enough even though the audience and fans wanted more. But if there had been another series for both I bet the audience numbers would have declined, only because a new generation has come to watch TV and all they like are shows that make people look stupid and reality shows that are the same thing over and over again. Outcasts was a victim of this generation, the show started off slow, I admit that, but it got better, the last few episodes were brilliant and it justified the show to have a second series, at least to tie story lines up. The show had the most intriguing cliff hanger, so many characters and plot lines just abandoned. The BBC really need to consider the audience that watched the show. They need to take into account the amount of people who watched the show on iplayer and who watched the re runs. I have never been so angry with the BBC for cancelling a show. I hope they redeem themselves.