In Season 2 Lord Zedd arrives and banishes Rita back to her dumpster prison.
While the Power Rangers movie was an all new version of the American kids’ show from the 90s, it based itself on the characters of the original Mighty Morphin Power Rangers, including the names of the Rangers and the villains.
Since the movie version saw Rita soundly defeated, it certainly leaves an opportunity for a new villain to step in, and Zedd makes sense as the one to do so.
To that end, it made sense for the movie to focus on Rita Repulsa as the major antagonist, as she was the big bad for the show’s first season.