In my humble opinion, it's the 'changes'.
The Kamen Rider or Masked Rider are always different in the yearly series, sometimes it's so creative that it breath of freshness, sometimes it's so drastic that it caused some winces. - these reaction are interchangeable and also even overlapped each other.
It's like a new version of the same thing that is not or never quite the same, and this actually help create more viewers and even potential fans, possibly even more than the numbers of the alienated long-time fans. Different concept would likely to cater to different crowd, allowing other people with different interest gateways to enter the franchise and possibly turned into actual fans. - It's like making up a bubble only to burst it yourselves and then remaking it anew... Or something like that.
But, why would such a thing work? Beside the factor of a totally new series every year, the other thing might actually be
Casting,
Gimmick and
Merchandising.
Casting pretty boys or girls might be awkward in the eyes of some, but that idea alone probably helped expanding the usual viewers especially the female crowd (housewives and OL), they might stopped once a series is over but some might instead become closet fans who continue to support the franchise by buying related merchandise like magazine or even OST.
It's pretty much the same when they cast pretty faces from boy-band or idol group, it helped by attracting viewers who are fans, and also opening up chances of grabbing some potential fans. - It's Japan, most of them grew up watching Ultraman or Kamen Rider, they might have dropped the franchise when they grew out of it, but a taste of nostalgia might lured some back.
Anyway, casting famous Voice Actor (VA) for Den-O and Kiva might helped opening a bigger can of tasty ham (

). the anime crowd is bigger than the usual tokusatsu crowd which included a lot more people who aren't really Otaku at all but the normal crowd who watch a few anime once in a while. - Potential fans, potential customer, every tiny bit of sand in the beaches count.
Gimmick, what to say, kids love them, adult love them too (maybe, especially the creative one), and the adult who are parents are actually the one who will have to spent money on them to satisfy their kids, and what guide them to spent more, new gimmick, every year!! New gimmick is also part of why every Kamen Rider is totally different from each other, you wouldn't want to miss out something new because other people is also watching (POV of a child).
Merchandising, now... Okay, it's a part of the gimmick. But, to be honest despite how greedy Bandai is or maybe (:evil: ), merchandising can actually work magic. - I know since I am knee deep, and yeah because I am pretty active on two or three group (ACG related - mostly Figuarts, Gunpla, Figma, Chogokin, Ultra-act and ect) on Facebook.
I have seen people who weren't into Kamen Rider or Ultraman (or dropped both franchise when they grew up) attracted to the merchandise showcased by other members, a lot actually started to watch Kamen Riders and also Ultraman of course, and some also started to collect the related merchandise. So yeah, merchadise can actually work wonder by attracting new fans who are unfamiliar or once familiar to the show. - Remember, most or all people in Japan and also South East were exposed to these franchise when they were young.
I mean if you look closely to the gimmick the people at Plex(Bandai) put into their design and concept, there are lots of hint or not-so-obvious related to other franchise or other elements. - There are educational elements like learning A to Z(Kamen Rider W), and learning about animals and color pairing (Kamen Rider OOO) and then there is learning about Astronomy (planets\star constellations) plus numbers in Fourze.
And what about the detective theme for W (hardboiled - also mentioned in Gintama), the alchemist for OOO (Fullmetal Alchemist Brotherhood was still airing), and the obvious PS buttons designs plus transformers style Power Dizer in Fourze.
These elements may not be as powerful as Hypno Toad, but hey they work. And yeah, again every little thing count.
And yeah, sometimes, the idea for the concept. casting and gimmick might be quite controversial, but controversy is actually the key of interest. People love them.
