During the 1980s, software companies were started, developing application types that are popular today. During this time there were several popular operating systems and computer manufacturers. When Microsoft started to dominate the market in the 1990s and early 2000s, many of the companies went out of business, or they were bought out.
For example, when the first Macintosh was released in 1984, its word processor MacWrite was one of the most popular on the market. However, in the 1990s, like many word processors it took a backseat to Microsoft Word.
Many applications were discontinued due to Microsoft Office’s market share, and many people and organizations did not migrate their old documents to active applications and formats.
As technology evolved, and they acquired new technology, many old documents could no longer be opened. This is one aspect of data rot, when data can no longer be accessed. A YouTube video gives more details.
Now there is much less concern over data rot.