Based on my limited knowledge of the series, changing the casts with every season was kind of a necessity. It probably made it easier for new fans to jump into the franchise if they didn't feel the need to rewatch previous seasons in order to follow along with the new one. It might have made it easier to use the sentai footage if they didn't have to create new explanations for the cast to get new powers too.
I'm sure that the decision was still a turnoff for a lot of fans, but I think that it probably helped to keep the franchise going instead of keeping the same characters around all this time.
I'm sure that the decision was still a turnoff for a lot of fans, but I think that it probably helped to keep the franchise going instead of keeping the same characters around all this time.