Film speed up.

[Mike Watterson]

Optical 50hz to 60hz conversion in either direction gives 10hz flicker amongst other problems.  This can be fixed by longer persistence phosphors which cause blurred motion. Colour conversion was almost impossible.

This is why stuff with USA contracts such as the original Avengers switched from 405 video tape to film. Thus only 2 episodes of series 1 survive and all the rest is scanned from film for DVD.