Unusual random thought on visual perception and movies: When visual images are missing element (like turn your head too fast) your brain fills in the missing time to make it seem smooth. That probably requires effort. I believe that 24fps is right at the edge of this effect.
I think it's one of those happy accidents that just so happens to hit this magic number that causes this effect in the brain. Like, it wasn't planned, they were just trying to save money cutting the fps as low as possible and found that was the sweet spot.