Pool film

Film is(was) made on Celluloid, as everyone knows.  But why?

When the film industry began, the very first films were recorded on a thin film made of paper.  This was clearly sub-optimal, because while the thin paper was appropriately transparent, it was also exceedingly fragile.  So they went looking for a synthetic alternative.

At that point, Celluloid (basically a mix of cellulose nitrate and camphor) was one of the primary commercially available plastics.

It had become commercially successful because the billiard industry at that time was running short of ivory for the making of billiard balls (conservation hadn't really taken a bite out of it yet, but they were honestly having trouble getting enough elephants to keep up with demand inspired by a billiard craze). The Brunswick Billiard company ^{1, 2} (probably?) ran a contest to find a synthetic replacement for billiard balls, and the Hyatt Celluloid Billiard Ball (or a variant thereof) was the winner in roughly 1868 ⁴.  I say probably, because apparently Hyatt never got the prize money ⁴, and the Brunswick company website ³ has does not mention the celluloid balls.  Possibly because although it was a much cheaper and easily acquired material than ivory, the balls themselves didn't perform as well.  Stories of the cellulose exploding are probably apocryphal, though.

Anyway, when the photography and then later the film industry went looking for their alternative, this material was no doubt an obvious choice, and the billiard ball factories were more than happy to supply it to a brand new market.

Sources:
¹ http://en.wikipedia.org/wiki/Billiard_ball#History
² http://youtu.be/7I0rX7t1J0Q (Adam Savage)
³ http://www.brunswickbilliards.com/our_rich_history/important_events/index.html
⁴ http://americanhistory.si.edu/collections/search/object/nmah_2947