Scotty is well versed in all things technical and, very likely, highly versed in the history of computing. See "The Trouble With Tribbles", in which his biggest desire with his given "shore leave" is to instead spend time in his quarters, catching up on his "technical publications". He's a man with a true passion for his field of expertise; which plays into his being a miracle worker.
I am taking at face value that voice commands are not as common in the time period in which Scotty works. It is shown in the show on several occasions, but diminishes in frequency in the feature length films. I am working under the assumption that the films took a more serious approach to the science in their science fiction (or some combination of that).
Being well versed in the history of computing and technology, he may have been expecting to encounter an old time-y construct used in technology, when every manufacturer became voice command happy. If his memory is like mine, and I assume there would be one or two similarities (I write web applications), he likely remembers that such a fad phase of tech existed "around the end of the 20th century", and not exact years or specifics of "rudimentary" devices. As we're still within the range of "the end of the 20th century" and people have long regarded talking to computers for command interpretation to be "futuristic", I believe this to be the most probable rationale (as @zipquincy suggested).
Also, comedy, as this is possibly the most cheesy* of the TOS feature films (per @calccrypto).
*Note: I only ever mean "cheesey" in a diminutive sense, if there is one, when it comes to the ST universe. Some synonym in the direction of "corny" or "light hearted" would probably be better suited.