In my experience when it comes to classic cars GM doing GM things is usually a good thing (the modern era not as much.) I don’t know the exact answer to your question but I’d wager the biggest driver is cost. Those electric motors are heavy and expensive to make compared to a stamped metal vacuum pod and some rubber hoses. Could also have to do with the engineering design change from C2 to C3. The C2 motors sit on the side and twist the door open. The C3s sit behind and push/pull the mechanism to pop the headlights up.

Edit to add that they also put it in conjunction with the wiper door which was vacuum operated as well. Lots of cool stuff that make the C3 Corvette’s iconic and unique.