Why Do All The Best Shows Get Cancelled??!!

Why is it that all the best TV shows get cancelled? 

Some of my most favorite shows in the history of television have had a series run lasting 3 season or less, but I still don't understand why networks decide to cancel shows that have such a massive fan base!

Veronica Mars:

This is one of the most amazing shows I have ever seen in my life! If I were a televion character this would be me, witty, intellegent and extrememly sarcastic, but also a crime-fighting badass!  Critics and fans alike are all disappointed in the fact that as her portrayal of Veronica Mars, Kristen Bell never recieved any awards from the academy.  I wonder if the death of this show had anything to do with the fact that any show that ends up on UPN has an untimely death (Buffy The Vampire Slayer, Roswell).  Kristen Bell as been lucky enough to go onto projects including Forgetting Sarah Marshall, You and Me and even the narration of Gossip Girl.

Tru Calling:

ELIZA DUSHKU.  Everyone from LA to the moon knows her face, the smart mouthed, fast paced thrill seeker Faith from Buffy The Vampire Slayer, but for the life of her cannot keep one television show on air... Tru Calling (an excellent show, featuring the likes of Zach Galifianakis (The Hangover) and A.J. Cook (Criminal Minds)) and Dollhouse, another Joss Whedon fan favorite starring Dushku as a human doll who can be imprinted with any personality depending on the demand of the client.  One of the most talented and diverse actresses I have seen in my short 21 years.  Tru Calling is the story of an average girl who has the gift of reliving days so she can save victims from their unitimely death, with the help of her wacky sidekick Davis (Galifianakis) and boyfriend Luc (White Collar's Matt Bomer).

The OC:

Ryan and Marissa.  Enough said.  These two don't even need an explanation, the love of the 21st century heard round the world.  Bad boy Ryan from Chino falls in love with OC Princess Marissa Cooper, entwining their lives with danger, violence and lots of wife beaters and leather jackets.  Unfortunately the show "jumped the shark" when creator Josh Shwartz made the decision to kill off Mischa Barton's character Marissa Cooper and make Ryan fall for the never-ending chatterbox, Taylor Townsend.

Roswell:

Based on one of the most influential and well known "UFO crashsites," Roswell follows four alien-human hybrids living in the town of Roswell New Mexico after their ship crashed 40 years earlier.  Their lives are mixed with romance, danger and mystery while they strive to figure out where they came from, how they get back and how they can survive in the process.  Roswell was the beginning to many names now widely known such as Emilie De Ravin of ABC's LOST, and Katherine Heigl of ABC's Grey's Anatomy and movies such as "The Ugly Truth" and "Knocked Up."