I've thought about this before, yet it was after recently watching Milk (Van Sant, 2008) and the Bones Brigade: An Autobiography (Peralta, 2012) skateboarding documentary that an idea seemed be be confirmed:
there is a 30 rule in cinema; namely, after 30 years a given historical topic is optimal for cinematic treatment in terms of audience receptivity and, likely, profits at the box office.
Both of the films mentioned above deal with the "living history" of the late 70's/early 80s -- moments prevalent in the minds of one generation and still accessible to a new generation interested in its past. After 30 years, filmmakers might enjoy both the advantage of historical hindsight and the ability to gauge the current reverberations of the event/s depicted.
Has this been written about previously? Perhaps there are correlates with fashion cycles? It's probably not an earth-shattering observation, and likely been written about before although I have not yet encountered it -- while I am aware of the 30 year rule in technology -- and I'm certainly not offering this 30 year rule as an absolute, for certainly any given era might be a subject for the camera.
But there remains a possibility that the 30 year rule exists. I know that personally, watching Mississippi Burning (Parker, 1988), a late 1980s film that represents the early 1960s, was quite formative for me in terms of developing a social consciousness as a teenager when I saw it in the early 1990s, and it roughly follows this 30 year rule pattern.