It's not a problem limited to movies, btw - plenty of music suffers the same fate.
"By the 1980s, it was becoming apparent that the collections of motion picture heritage were at risk of becoming lost. Not only was the preservation of nitrate film an ongoing problem, but it was then discovered that safety film, used as a replacement for the more volatile nitrate stock, was beginning to be affected by a unique form of decay known as "vinegar syndrome", and color film manufactured, in particular, by Eastman Kodak, was found to be at risk of fading. At that time, the best-known solution was to duplicate the original film onto a more secure medium.[5]
A common estimate is that 90 percent of all American silent films made before 1920 and 50 percent of American sound films made before 1950 are lost films.[6][7]"