It's tough for a TV show to keep up that initial quality and momentum as the years fly by. Which is why so many shows start out great, and then - as the seasons progress - they flounder in an attempt to keep audiences hooked.
Sometimes a show just exhausts its premise, plain and simple, and trudges onwards purely to score ratings. There's no shortage of shows currently on the air that are well past their prime, after all, and there are plenty of shows that people still tune into purely out of some misplaced sense of guilt - like they owe it to the writers to see the story to its end.
When a show genuinely begins to go downhill, then, it's rare that it will turn things around before it's too late; seasons become progressively more desperate and less compelling, until - boom! And by then either cancellation has set in, or the fanbase has dissipated to the point where everybody who once loved or cared about the show has moved on.
Sometimes, though, a great show can experience a brief period of decline, only for it to unexpectedly rise from the ashes to deliver some of its best seasons or episodes ever...