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)
Movie was made only to give closer to the unanswered question.
Here creators word on the whole story.