Because five movies were not enough. This was essentially a re-imagining of the original movie, continued as a series. It doesn't really fit into the continuity of the films; although vague references are made to astronauts that visited '10 years ago.' The humans in this version speak, unlike the films. The series ran from September 13, 1974 to December 20, 1974; although fourteen episodes were produced, only 13 were originally aired; the 14th previously unseen "lost" episode was included in the DVD which was actually released. I could not find the actual opening, the clip is a video tribute to the series.