Of course I don't know why they had to out-continuity her in TVH, since she's been shown in the comic series' as being a great character. In DC's run she practically became part of the crew's family!
Perhaps Nimoy and/or Bennett didn't enjoy the experience of working with her and/or her performance in Star Trek III
. They couldn't not bring her back in Star Trek IV
given where the film starts and where we last saw the crew, but that also doesn't mean they had to give her a meaningful role. In that case, they gave her the bare minimum to explain where she's gone, and that's that.
The other possibility. Nick Meyer wrote the San Francisco half of the script. He didn't want to work with Curtis years later on Star Trek VI
; perhaps that went back years earlier and he had no interest in writing for Curtis' Saavik on Star Trek IV
. If that's so, then Meyer's half of the script didn't use her, so Bennett had to write Saavik out to explain why she's not there in the middle half of the film.
Or, they simply couldn't come up with anything for the character to do in Star Trek IV
, so it was easier to write the character out for the film than have the character hang around like a useless fifth wheel.