Yep, I heard that Denny Martin Flynn (or whoever the director was) had no interest whatsoever in working with Curtis.
The director was Nicholas Meyer, who had previously cast Kirstie Alley as Saavik in THE WRATH OF KHAN. He did not direct the two films with Curtis. (Nimoy did.)
Having Saavik be the traitor would have been more dramatic, I agree, but can't blame Cattrall for not wanting to be Saavik #3--if that was indeed her call.