The 1970s superhero TV show, Secrets of Isis, was cancelled after two years. According to the Wikipedia page it was popular. Why was it cancelled?
The show was got canceled due to low ratings.
As mentioned here:
The Secrets of Isis was quite possibly the first American live-action TV series about a female superhero – predating both Wonder Woman and Bionic Woman. It debuted on CBS in September of 1975. All in all, two seasons and 22 episodes of The Secrets of Isis aired before the show got canceled due to low ratings.