Kirstie Alley was either uninterested in returning or out of their price range (I forget which), Meyer was uninterested in using Robin Curtis and Kim Cattrall didn't want to be the third actress to play Saavik. So they made her a new character.
Honestly, I'd put a lot of that at Nimoy's feet. Let's be real, Kirstie Alley's Saavik was intended at least partially as a replacement for Spock (an attractive young actress, new blood in the franchise, and playing a Vulcan who's conflicted about her dual heritage). If Nimoy was coming back to Trek on an ongoing basis, it's no wonder he wouldn't want this new character around for much longer, taking up screen time and storylines that arguably could've been Spock's. So I think it's very possible that Nimoy intentionally directed Curtis to be rather wooden as Saavik, which killed off a lot of interest in the character.
I definitely heard the story that Kirstie Alley
wanted to be in
Star Trek III, but she or her agent asked for too much money and blew the negotiation. There was a
Starlog interview where she said (approximately) "She's not Saavik, I'm Saavik!" in the headline. Alley didn't reject the role, she missed out on it— that's what I understood it to be.
It never occurred to me that Nimoy wanted
Robin Curtis to fall flat in ST3, but it makes sense. He did give her specific instructions to be unemotional and have 5000(?) years of Vulcan wisdom behind her eyes, to reflect some baloney about
Vulcan katras being passed on cumulatively forever. And that ran dead against not only the ST2 Saavik personality, but Spock himself. Spock was never serene and infinitely wise, he was a guy
seeing life for the first time and dealing it, like the rest of us.
So making Curtis's Saavik seem
above it all, unengaging and dull, not only hurt the character, it contradicted what had come before. An "unexplainable" mistake like that can often be explained if there is a hidden motive. Nimoy may have thought Spock came across as the "second most interesting Vulcan" in
Star Trek II, that he was upstaged by Alley's fresh, vivid, realistic character, and he didn't want that to happen again. So yeah, I can see the possible motive.
BTW, Robin Curtis has always been linked in my mind with
Robyn Douglass (
Galactica 1980). I think of one, I think of the other. It never fails. They're like the Susan Anton and Morgan Fairchild of science fiction.
