If you're wondering why there wasn't nitrogen mixed in, it's because the additional systems+tanks for handling nitrogen would've added more weight to the spacecraft.

This video goes in to a bit more detail: https://www.youtube.com/watch?v=FvA7N_j_8os

Reducing the pressure reduces the forces on the pressure vessel. I assume that only the oxygen partial pressure is actually important for respiration.

These two articles talk about this question:

https://www.popsci.com/why-did-nasa-still-use-pure-oxygen-af...

https://www.space.com/17338-apollo-1.html