The medium of film is quite fragile. It. An decay and be destroyed quite easily. And it's expensive to reproduce and store. We've lost countless important video items even from the 1980s because it was improperly stored or destroyed because a person didn't want to pay to store it anymore. For example, the original film of the TV show MASH (1972-1983) were lost because they were improperly stored and decayed. Copies of copies had to be used for video, and several minutes from each episode were lost. Many original Doctor Who TV episodes were simply taped over, despite it being incredibly popular. The original moon landing footage was lost. And no recording of the first Super Bowl exists (just recently, a portion was found to exist in storage in poor quality).

Meanwhile, paper is more durable and easier to reproduce and store, so musical notation is a lot easier to preserve. Nevertheless, a lot of the works of Mozart, Bach, Brahms, Beethoven, and others has been lost, because people didn't bother to reproduce them. A very large portion of written works that existed before the printing press was invented, are lost forever. Several of Shakespeare's plays may be lost (or maybe not--one work is Love's Labour's Won. But it might still exist but be a different name of a play we already know). Also, even though musical notation has been passed down, the music we know today might sound quite different from the way it sounded when written centuries ago. Notes have become more sharp. Musical instruments have become better able to stay in tune, and tempos might be different, among other issues.