Because of low TRP , it got critical acclaim but it lacks in attracting more viewer. So it got cancelled after 3rd season.

>  A group of fans calling themselves the "Cloud Watchers" sent more
> than 10,000 Mars bars to the CW, hoping that the network would reverse
> its decision and renew the series.(Source:[Wikipedia][1])


 Movie was made only to give closer to the unanswered question.

[Here][2] creators word on the whole story.


  [1]: http://en.wikipedia.org/wiki/Veronica_Mars_%28season_3%29#Cancellation_and_future
  [2]: http://www.washingtonpost.com/blogs/style-blog/wp/2014/03/13/veronica-mars-creator-rob-thomas-explains-how-his-canceled-show-rose-from-the-ashes-for-a-groundbreaking-movie/