Star Trek, Behind The Scenes

The series was canceled in its third season, despite the protests of a renewed letter-writing campaign. Marketing personnel of the network complained to management that the series' cancellation was premature. New techniques for profiling demographics of the viewing audience later showed that Star Trek had been highly profitable for advertisers, though this news came too late to resume production of the series.
Rebirth
When the show was cancelled, owner Paramount Studios hoped to recoup its production losses by selling the syndication rights to the show. The series went into reruns in the fall of 1969, and by the late 1970s had been sold in over 150 domestic and 60 international markets. The show developed a cult following, and rumors of reviving the franchise began.