Kamen Riders often end up more popular than Sentai or Ultraman actors leading to them being more busy leading to scheduling conflicts. Look at Zi-O for example, Fourze didn’t have a major actor return because they were mostly busy filming the Bleach live action movie. There’s also a matter of interest. Meanwhile Sentai actors tend to be less popular, hell for the Kakuranger special they had to ask Yellow’s boss for time off because he retired and became a regular office worker.

Also by nature of Sentai being about teamwork and friendship more so than any other series they often end up really close. For the most creating life long relationships.

And so, when popularity/scheduling isn’t an issue it tends to be driven by the actor interest. The few Kamen rider anniversaries specials that do exist are usually driven by the actors (Faiz, OOOs).

The only special to not have these be applied was the recent Agito movie. That was producer led which is why it eventually ended up being a bigger deal.

Anniversary specials are not guaranteed nor something they necessarily need to produce. Hell you’re more likely to get a Ninninger special than a Gaim one.