Mighty Morphin Power Rangers, Season 1.
Originally, MMPR was expected to be sort of a "one hit wonder." Saban had for years been trying to bring Sentai into America one way or another. His first attempt was a Bioman dubbed pilot that aired in 1986 but went nowhere. After that, he had tried pitching his idea for Power Rangers using the footage of first, Turboranger and later Jetman to American TV networks but he couldn't sell his idea. Then, Fox Network, which was new at the time, decided to give MMPR a shot. Saban used the Zyuranger footage instead of Turboranger and Jetman. At the time, Fox was anxious to try anything, but both Fox and Saban thought MMPR would be able to only last a year at best. The original ending of the series was suppose to be the two part episode, Doomsday. However, MMPR had quickly became the biggest kids show on television, which forced Saban to alter his plans for ending Power Rangers after one year, and extend MMPR S1 by about 15 episodes. To do this, he borrowed money from Fox and used it to ask TOEI to produce more Zyuranger footage that would be used only for Power Rangers. This footage is now famously called Zyu2.
Power Rangers in Space
The second time Power Rangers avoided cancellation was Power Rangers in Space/Turbo. Every since MMPR S3, the ratings had seen a gradual decline, although they were still serviceable. But in Turbo, the ratings took a free-fall, particularly the first half of the season. The second half of the season the ratings gradually went up, but the ratings were still dismal and below that of Zeo's. At the time, both Fox Kids and Saban were losing money from the show's presence, so Fox told Saban to make Power Rangers in Space to be the last season of Power Rangers. This is why the Zordon Era ended and why we had the finale "Countdown to Destruction." It was thought that PRIS would be PR's last season. However, PRIS like MMPR, was a big hit and its ratings were in fact, on par with MMPR. Because of the success of PRIS, Fox decided to let Saban continue with Power Rangers, and thus, Saban cancelled the second season of The Mystic Knights of Tir Na Nog to focus its entire budget on PRLG.
Power Rangers Wild Force
After Wild Force, Power Rangers again was suppose to be cancelled due to the Disney buyout. However, former PR producers and writers Doug Sloan, Ann Austen, and Jackie Marchand were all working for Disney at this time. They successfully convinced Disney to keep the show running, and as a result, Sloan, Austen, and Marchand were the main producers of Ninja Storm and Dino Thunder.
Power Rangers Jungle Fury
After Bruce Kalish left and abysmal ratings, Jungle Fury was intended to be PR's last season. Disney didn't want to deal with finding a new producer for Power Rangers, and there were heavy talks about Disney switching to an animated series that would be based off of MMPR. However, TOEI disapproved of the animated series because it meant that they wouldn't make money off of the Sentai footage used to produce Power Rangers. Bandai also disapproved. However, due to outside forces (mainly because PR was still very popular in Germany), Bandai and Jetix Europe gave Disney enough money to produce one final season, which would be RPM.
Power Rangers RPM
This time, after Power Rangers RPM ended, the series was cancelled. Disney was tired of splitting the money with Bandai and TOEI, so they decided to re-edit MMPR instead, which was among the worst of Disney's PR projects. RPM had the worst ratings ever for a Power Rangers series, however this was more due to being screwed by the network than lack of popularity. RPM was moved away from cable tv onto ABC, and was placed in the worst time slot imaginable. In some parts of the country, RPM aired at 5am in the morning, and entire episodes were not seen due to being subject to ABC's sports coverage. However, as we know now, Saban bought back Power Rangers from Disney, and immediately decided to produce another season.