Even putting aside the fact that the TMP script itself hadn't been tinkered with for that decade, various versions of "In Thy Image" had been floating around for the better part of 3/4 years by the point it finally went in front of the cameras. And they were still
revising it while it was being shot!
I blame the confusion re: Phase II. The production psyched itself up for TV, and when the decision was made to green-light it as a movie instead, it caught everybody flat-footed. Everybody was rushing to adapt what had previously been a television script into something much grander, which is why the actors were receiving fresh pages on a daily basis that mostly consisted of scenes where they react to V'ger on the viewscreen. The idea was that maybe they could upscale it to movie size with lots of spectacle, but even then the effects house was being pushed against it's limits. Hence, alas, the memory wall was abandoned...
(I wonder if any of Spock's space walk as eventually seen in the movie was actually salvaged from raw footage shot for the memory wall?)