Probably because Robert Wise was not motivated by nostalgia. His brief was to reinvent Star Trek for the big screen, to rethink every aspect of it.
Besides, the reason Roddenberry cast Barrett as the computer voice was probably because she was his mistress and he wanted to include her (or she wanted to be included) in everything he did. The recurring voice role let him do that in episodes without Chapel. But Chapel was in TMP, so there was no need to double her up as the computer.
Note that none of the first six movies used Barrett as a computer voice, aside from the TMP thruster suit. It was Marcy Vosburgh in TWOK, and TSFS uses several different Starfleet computer voices, including future DS9 computer voice Judi Durand, Harve Bennett, Leonard Nimoy, and Nimoy's assistant Teresa E. Victor. (So it seems like they pretty much grabbed whoever was available to loop a few lines and didn't consider voice casting all that important.) The reason we think of Barrett as the "essential" computer voice now is because she reprised the role in TNG onward. At the time the original movies were made, that perception wasn't there yet, at least not among the filmmakers.