Actors lost their looks/ wanted too much payment/ quit/ disappeared,
New boys work for peanuts
And the fact that the don't like working on shows like this. Some of those actors consider superhero roles to be an embarrassment. (I remember, for example, that Joe Odagiri did not like doing
Kuuga.) Throughout the Heisei Rider Series, Toei has liberally casted "here-today-gone-tomorrow" idols, few of whom have any interest in the Masked Rider franchise. Just picked out for their looks.
When I saw the promo for
Decade, to see what the big deal was, I noticed that someone else was playing Godai/Kuuga! It took
GUTS for Toei to recast for a hero character, when in the past, that was
very rare! (That character would often either be phased out or killed off, because a character was tailor-made for an actor, so they conceive a new character for a new actor.)
Tsuburaya Productions is very careful about the actors they choose for the Heisei Ultramen. Especially because of Hiroshi Nagano (which was not his fault, but his manager's). These guys have commitment.
I just hope that these new actors have that same commitment that most of their predecessors sorely lacked (regardless of how I feel about the Heisei Rider Series).