system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come a lot of films from the 1920's and older have become lost, yet a lot of films from the 1930's and forwards are still in existence (despite the continuing use of nitrate films and the lack of need for preservation at the time)?
gpt
Well one big reason is that for a very long time, film preservation was not really a thing.  Once a movie was screened at a theatre and it was done, the movie was considered worthless and was scrapped or reused because film is expensive.  Remember, there wasn’t a TV or anything to resell your content to and that meant that if you wanted to save your movie, you had to go through the effort to do that and techniques back then were not good.  

You mention the 20’s even though movie destruction has been a thing long after the 20’s (a famous example of lost content is old Dr. Who TV episodes) one thing that happened in the 20’s was the great depression.  Lots of companies including movie companies went out of business and movies were just tossed when the doors were closing since they couldn’t afford to store them.

ETA:  Film preservation is a really modern concept.  Lots of movies are partially or outright gone forever for various reasons.