I notice that Sentai has re-used several theme (dinosaurs, ninas, etc) throughout its run, while KR has a new theme every year since the Heisei era. Is it because the team-structure of Sentai limiting which theme can be chosen?
My personal theory is that the themes that get used over and over (three dinosaur team, three Ninja teams) are ones that are very popular with the target audience, children.
Children tend to be really into things for a while and then grow out of them with only a few hardcore fans sticking with the shows they liked as kids into adulthood. That means a constant revolving door of an audience who will be really, really into something for 5-10 years and then move on to something else. As a result, what may seem rehashed to us older fans who have stuck with the Super Sentai series year after year would be fresh and new for the members of the target audience just being introduced to it for the first time.
So, repeating a theme that had proven popular in the past for a new audience makes sense from the point of view of marketing.
When it comes to Kamen Rider though, the audience tends to skew a bit older. The series is written to appeal to almost all ages, with more complex plots, dramatic moments and ongoing stories that take multiple episodes to resolve. It doesn’t have the same kind of revolving, aging-out audience that Super Sentai does.
This is, in my opinion, why Kamen Rider has to work to come up with fresh, new ideas every year and almost never repeats a motif. The fans don’t grow out of it and would get bored if it kept doing similar things over and over. Kamen Rider makes more money for Toei right now and they can’t afford to rehash things in fear of losing ratings and toy sales.
However, this is just my opinion based on a little research and my own thoughts on the matter.