#The katra refusion belief seems to have been somewhat limited
It's not clear how much of Vulcan believed in katra and its various uses, but it's clear that the idea of rejoining body and soul was a belief most Vulcans did not share. We've not seen any other Vulcans use katra like Sarek and Spock have. Even Sarek himself wasn't sure it would work.
SAREK: I ask for fal-tor-pan, the refusion.
PRIESTESS: What you seek has not been done since ages past, and then, only in legend. Your request is not logical.
SAREK: Forgive me, T'Lar. My logic is uncertain where my son is concerned.
#Spock's body was radioactive material
One of the major problems with trying to bring Spock back in general was that his body had absorbed a lethal dose of radiation. That Spock dies within minutes speaks to how much radiation he was exposed to. Even if she has been a believer in katra refusion, Spock's body was far beyond any use. It's only after having been regenerated on Genesis that the refusion was even an option.
#She might not have known Spock did the katra thing with McCoy
Saavik was on the bridge when it happened. By the time Sarek confronts Kirk, Saavik had already been reassigned to assist the USS Grissom in investigating Genesis. It would make sense that she didn't have time to review it herself.