Well you would need twice as many feet of film for 48fps, and it would need to be fed twice as fast, with light flickering twice as fast, etc. I assume that wasn't easy to do.

Film was shot at 24fps, a compromise between limiting motion effects while saving on film.

Each frame is actually projected twice in a film projector because you would see flicker at 24fps so the brightness/number of images is the same. It's just that at 48fps each is different