It probably has nothing to do with time and everything to do with pay. Kamen Rider tends to use relative unknowns who can be worked pretty hard for low wages. Then when a former Rider actor like Hiro Mizushima hits it really big post-Rider, he's in a position where his time is more valuable and he would rightfully expect to be paid more.
Toei clearly doesn't like to spend a ton of money on actors since the ROI isn't good-- for a Rider show it's best to spend a ton of money on effects. So I seriously doubt Toei could pay for more than a brief cameo for Mizushima. Paying a now-standard rate for someone like Odagiri who has become a legitimate superstar post-Rider is just not likely.
I'm sure we're getting Wataru showing up a lot since his Rider debut was just last year-- and that means his prices haven't had much time to increase in the meantime, so to speak. Likewise, I don't think the actor who played Blade ever really took off as a big star so he would probably still be affordable.
