I started thinking maybe the Kodak Instamatic, Pocket Instamatic and Disc cameras took market share from the people who would have shot half-frame.
But then I hit on something.
It must not feel right to shoot verticals. Now I'm not saying it's wrong to make a camera with a natural portrait orientation. I know the mark of a professional is a good variety of portrait and landscape shots that would make it easier for the Art Director laying out magazines. Not every shot can be a double-truck.
But if half-frame cameras had been made to comfortably shoot in horizontal orientation, it might have taken off.
It might have been exciting to be able to mount motion-picture lenses on your still camera and get essentially the same shots the cinematographers were taking.
Think about it, nobody makes a vertically-oriented d camera... even though there's no mechanical constraint.