Great question — answering as the UphillTech team:
As Catija mentioned in the comments, in the late 1920s/early 1930s, feature-length films were likely becoming increasingly prevalent due to improvements in movie-making technology. Indeed - again, as referred to in the comments - the first feature-length talkies began to come out, starting with The Jazz Singer in 1927. (For a more detailed review of film firsts, you can take a look at my answer to an older question here - you'll notice that the 1920s get mentioned a lot .) As for the spike you see in the 1980s and '90s, there may be some bias due to the criteria for inclusion on the graph (namely that at least one imdb user had to have rated it, and many users may be more familiar with '80s...
Full answer by ghostdog: https://movies.stackexchange.com/a/62846 (CC BY-SA 3.0).
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