Why do they show movies at 24 fps (?).
The quick & dirty answer to that is $
It costs a good deal more to up the framerate 25%. There's 25% more film being run through the projector, there's 25% more film to produce, there's 25% more film to ship, etc., etc., etc., etc.
Theaters hardly have a budget to keep things running (most aren't running properly), let alone have a budget for the added cost of 30fps. And the public doesn't know their ass from a hole in the ground as far as movie theater presentations go. So why should they bother to spend any $ to give a better presentation, when the public won't even notice.