Well lemme remind you a few things about nintendo. As you know this company had a massive lead during the nes/snes era and they asked impossible stuff to developpers like:
- very high royalties
- many exclusives games
- sometimes they wanted a independant dev to work for them, and only for them.
If you add the specific format of the N64 that was pretty dislike at that time, you understand why sony had so much 3rd party support.