Remember in 1990 (the pictures are from around then) there wasn't unlimited fast disk space. There was a good chance you were saving to a floppy disk which not only was slow, but might not be in the drive all the time. At least in 1990 you could assume a hard drive - but you couldn't safely assume there was enough free space on the drive to save anything, since 40mb drives filled fast.

35 years latter we live in a different world where you can assume an unlimited fast data connection to a data center. Saving and allowing undo is possible today.

Even in 2005 when I was a kid using computers in school, I had some larger docs where constant autosave would've been unreasonable. Word had a timer that was like 5 minutes.