Infinite TV series

Why when you like a TV serie so much, and you think it's original and fine, they spoil it?

Let´s think:

Alias: It started so well, it really trapped you.But when the scriptwritters were run out of ideas, they started to repeat the same plot line (Sydney wants Vaught and can't have him, they beat SD-6 but another same-kind organizations appears and when they beat this new one, another shows, etc.)

 House: I'm tired of House! At first it was something surprising but now every case are the same. They found the patient a disease, they try to cure him, the medications produces some new disease... it's boring!

Dawson´s Creek: Could anybody see this serie till its end? It's soporific!. The way they talk and the plot line are surrealist.

What it's worst, the spanish tv series, it not only last a never-ending 1 hour (or more with the advertising) but the plot lines are very fine the first season and totally bad the next ones. Why? They want to earn more money squeezing a product that no longer is a success. They can't think of new ideas, they live on their first idea for the rest of the serie's life. A lot of spanish tv series enden without anybody of the original crew. It's a totally different serie!