Power Rangers can be (pretty firmly) delineated between it's first 7 years, and its second 8.For the first 7 years, there was, to one extent or another, a continuing cast to the series. (and continuing story, but I'm focusing on the cast.) Characters swapped out in 1's and 3's, left the show and later returned, or promised to return and never did because everyone thought the actress was a colossal- never mind. A slowly rotating organic cast.
Then, beginning with Lightspeed Rescue, a new phenomenon emerged- new casts, every year. New location, new casts, new plot, completely self-contained.
The series prior, Lost Galaxy, also had a completely new core cast, but also had Bulk and Professor Phenomenus from the prior year, Alpha 6, and added Karone from PriS midway through. PRLG is not the 'island' that later series were.
Of course, the big secret here is- this big stylistic change wasn't exactly intentional.
The Power Rangers Lost Galaxy cast was signed to 2 year contracts. (Well, 1 year and an option.) If they worked out, they'd be heading back to Earth, or Lightspeed would take place on Mirinoi... or something. The option for a second year was left open. In the end it wasn't exercised. PR's ratings were down- it was being slaughtered next to Digimon which was receiving all the advertising time and weekend marathons preempting LG, and it would be difficult with the way the series ended to just return them to Earth. The official decision was- "We'll only stick with this cast if we can get all 5 of them." Valerie Veronon was undergoing Leukemia treatment, and that wasn't going to happen.
Lightspeed Rescue had one of the most stilted, turgid casts is PR history. Which is fine, because while I'm betting they also had the 2nd year option on their contracts, it was clear by the time the series was 1/3 over that this was considered a time-killing year while waiting for NEXT year, with it's beautiful-looking Sentai... The Lightpseed cast could have continued, but was not going to.
Ah, Power Rangers Timeforce, beautiful Timeforce. Their zords have a timeship mode, and they have only 1 main megaozrd, not the hordes that plagued adapting Megaranger into Power Rangers in Space! Chip Lynn (series writer) showed clear elation. Press releases delighted in visions of the Power Rangers traveling back and forth in time for adventures! PRiS had taught them the trick, any Megazord fight in a natural setting could be declared 'the past!' This was PR's big push after a few years, a bigger budget, a longer productions schedule- there were going to be as many episodes of PRTF as there were of it's Sentai! And a MOVIE! There's going to be a MOVIE! Surely, this cast will continue on to the next season, by hook or by crook if they're in a movie!
Sadly, Chip Lynn discovered that the Japanese are absolutely insane. The 'Timerenjaa' never traveled in time (despite series credits showing them doing so.) The series was a riff on 'The MAtrix,' with lots of bullettime gags, and was a dense urban soap opera. There WERE no natural setting Megazord fights that could be used for past locations, they ALL took place downtown.
A threatened SAG strike scuttled the movie. Wiser heads concluded this was not the year to put the money into PR's 'big push.' The production schedule was savagely cut back- leaving a series that had been proceeding at a leisurely pace with its plots suddenly missing 16 episode at the end when it planned to tie everything together. It simply dropped plots, and quietly adapted the Japanese finale, including a down-beat ending that the US characters has been specifically conceived of to escape. Chip Lynn quietly left the series in disgust.
Power Rangers Wildforce's cast was never intended to go more than a season. A deliberately conservative year to 'season' the new series writer, it adapted its source Sentai in great style, simply dropping the ancient kingdom of Aimaria into the timeline of PR's backstory and (for the most part) producing a point-by-point translation of Gaoranger. Wildforce, however, also gave us Forever Red, the 10 Red-Ranger teamup with all the old actors returning. The NEXT year ('Ninja Force' is the my term for 'Ninja Storm as conceived by the crew of Wildforce') was going to be huge- a new core cast, but old cast from many previous PR seasons returning. Tommy in a mentoring role, appearing multiple times throughout the season, etc.
Disney bought the series, fired everyone, and moved the show to New Zeland where it could be filmed cheaper.
This was really the nail in the coffin for PR's continuing casts. MMPR Productions, much as they had produced 4 years of PR with new casts each year, always knew they WANTED a new continuing cast. But now everyone from MMPR Productions was gone, and the new crew looked to previous years for guidance on how to do things, and concluded that Power Rangers 'means' a new cast each year.
It's been this way for 7 (well, 8, we know Mystic Force isn't SPD's group, so) years now. And the wholly unintentional pattern of PR constantly changing over its casts now has huge inertia behind it, to the degree that the first writer who tries to undo it is going to face incredible friction- why isn't he doing what PR's always done?
It wasn't supposed to be this way. I think it hurts the show.
There it lies.