One fundamental difference between TV and film is that in TV there is, theoretically, no end. A film is conceived with a beginning, middle and end — any basic manual on scriptwriting will tell you so. Barring sequels, that’s the end of the road for a movie. You tell your story in that framework and move on. In a TV series, each episode has a beginning, middle and end, but the story is not over until advertisers no longer want to buy time. Finality only comes when enough people stop watching, not when it makes dramatic sense. This has resulted in the inhumane imprisonment of countless sharks in tiny swimming pools with only a regular procession of bikers flying over their heads for distraction. Somebody call PETA.
When quality TV goes bad, what usually happens is that the audience maintains an emotional attachment to the characters that keeps them watching when there is little in the way of creativity or artistic merit left. George Lucas excepted, movies can rarely fall into this trap. Effectively, the TV show turns into a soap opera; no fully-baked plots, just following the characters through their trials and tribulations for as long as your emotional attachment lasts. The audience dwindles over the ensuing years until all that’s left are those deeply disturbed types who celebrate the character’s birthdays and take plot discontinuities personally.
Another difference is sheer quantity. Producing TV shows requires awe-inspiring creative endurance. Your standard hour-long broadcast TV show has 26 episodes in a year, each roughly 40-45 minutes in length. That’s pushing 20 hours of drama or the equivalent of 10 movies or stage plays every year, with flashback episodes as the only respite. At that level of quantity, quality must almost certainly suffer.
That’s what makes shows that have been long-term artistic successes so impressive; the core seasons of shows such as Seinfeld, St. Elsewhere and Northern Exposure come to mind, but even they were ragged on either end or the run. The only show I can think of that did not continue longer than it should have was Fawlty Towers, the great John Cleese farce for the BBC in the ‘70s. Thirteen side-splitting episodes and they were done. And as sour as Cleese’s career has turned over the years he has never attempted a revival. It remains pristine.