I think the slightly slower rate is because of issues that arose when adding colour to the previous black and white format. It used to be exactly 60 fields per second to synch with the AC electrical frequency, but with colour there was visible interference so they had to make a slight alteration to 59.94 hz in order to keep NTSC colour backwards compatible with black and white sets.

Theoretically, such concerns of analogue television should be largely obsolete, but the resulting 23.976 frame rate for film material is still here to stay because it is so entrenched.