Great question — answering as the UphillTech team:
There are two related reasons. The first is that feature film production, from the 1920s onward when film cameras became standardized, was entirely designed around working at 24fps. Cinematographers learned to exercise fine control over exposure, image quality and depth of field by balancing shutter angle (how long the frame exposes--literally how wide the gap or gaps in the film camera's spinning shutter is), f-stop (the width of the iris inside the lense), film stock (faster but grainier vs. slower but cleaner) and filters (at a minimum, neutral density filters, which reduce light transmission to the film without altering color). There are some very complicated tables and a lot of rules...
Full answer by SpaceToast: https://movies.stackexchange.com/a/97295 (CC BY-SA 4.0).
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