Because of low rating, it got critical acclaim but it lacked in attracting more viewers. So it got cancelled after 3rd season by The CW.

> 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])


The movie was made only to give closers to the unanswered questions. Even Warner Bros. said no to produce the film, it was [Kickstarter fund-raising][2] which made it possible.

[Here][3] is the creator's word on the whole story.


Note: So if you want to complete the story you need to watch the movie after the 3rd season.


**Edit-**

As  BCdotNET said there is a [Season 4 teaser][4] made for an attempt of making season 4 but this teaser is became non-canonical because of movie’s story conflicting with it.


  [1]: http://en.wikipedia.org/wiki/Veronica_Mars_%28season_3%29#Cancellation_and_future
  [2]: http://en.wikipedia.org/wiki/Veronica_Mars_%28film%29#Development
  [3]: 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/
  [4]: http://veronicamars.wikia.com/wiki/Veronica_in_the_FBI