A Marilyn Monroe clone. Writing is one of the biggest factors. Often as soon as the original writers pack their bags, the show goes straight down the toilet. An important cast member moving on to greener pastures, in a bigger show or in movieland, can also be a disaster. Ratings-hungry executives can be the most devastating, reaching in and stirring things up on a perfectly good show.
Some programs survive the trials.
Many good shows have died an anonymous death this way. Humans are creatures of habit, and if they watch a show on Thursday, chances are good that they won't follow it to Tuesday. They might not even be able to. If their shows get moved to different nights, they simply can't watch them anymore. And so we say goodbye to some of our favorite shows.