A major issue, that affected finding "Raspberry Pi assembly" in the 1990s was that the Raspberry Pi didn't exist yet.
A lesser issue, and perhaps one that may have made it difficult to figure out how to "write an OS (just for fun)" is that few people had done this and published tutorial-style material for absolute beginners on the web- and most of those tutorials that did exist weren't on the world wide web (but on e.g. usenet).
A major issue, that affected finding "Raspberry Pi assembly" in the 1990s was that the Raspberry Pi didn't exist yet.
A lesser issue, and perhaps one that may have made it difficult to figure out how to "write an OS (just for fun)" is that few people had done this and published tutorial-style material for absolute beginners on the web- and most of those tutorials that did exist weren't on the world wide web (but on e.g. usenet).