I think part of it might be that, the ones the preceded Black are a bit to dated and campy to really take seriously. Not that they can't be enjoyed, but Black's efforts to tone down the campiness and give it a bit more of a story, probably made it easier to get invested in the plot.
Black came five years after the previous series, so the franchise basically went from 70's/early80's sensibilities, to late 80's sensibilities.
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Apologies for the double post but I felt like keeping up to date with what series they are adding to youtube.
And this week it's the 2002 series Kamen Rider Ryuki (guess they aren't going in order).
Ryuki is quite a unique series and is somewhat of a deconstruction of the franchise. As oppose to previous entries that either focused on a single rider or occasionally two or three, Ryuki had a whopping thirteen riders. The story also added a lot more moral ambiguity to the title of Kamen Rider, as the riders of this series is are all stuck in a deadly battle royal wit each other were only one can be left alive. To add to that Ryuki also was the first season to introduce the idea of evil Kamen Riders to act as foils to the more heroics ones, as several participants in this battle royal, are most certainly bad guys.
These traits made it somewhat controversial among the fandom, with finding it a fascinating deconstruction of the franchise, while others found it far to much of a departure, while others didn't hate it's concept but found it's exicution to be a pretentious meandering mess. One way or the other the idea of evil riders would become a staple of the franchise after this.
Anyway with all that in mind here's the first episode of Kamen Rider Ryuki. Hope you enjoy.
https://www.youtube.com/watch?v=CrO7vBqZaFQ
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Well continuing with the random order of Kamen Rider stuff being uploaded, we know have the first episode of the 1974 series, Kamen Rider Amazon.
A rather unique series for it's time, it broke away from the previous formula, of cyborg who fights against terrorist organisation (as had been with the prior three series) and instead has it's main hero be a Tarzanesqe wildman, who's abilities were more mystical in nature.
It is probably most famous (or infamous) for being much more gory than the other entries in the franchise. This was done mainly to get across the idea that the titular rider had more feral and animalistic fighting style than his predecessors. And as a result he would violently dismember his opponents in battle.
https://www.youtube.com/watch?v=lUIq-FIQ0oA
Also guys if you want me to stop with this please say so, I don't want to clog up the place with a thread no ones interested in.