[Hegewisch Dupa]

The thinking was that if you are going to make the capsule atmosphere 80% nitrogen like Earth's, then you are adding a lot of weight for something that is completely unnecessary. Except that yeah, it was only a good idea on paper, and nitrogen's inherent ability to prevent fires starting was a pretty damn useful thing.

Another problem with the Apollo 1 fire is that when on Earth, the capsule pressure has to be much, much higher to prevent the capsule from collapsing under the pressure of Earth's atmosphere. So the oxygen concentration was far higher than it would have been in space.

[IronJack]

As I recall, another problem was that the hatch to the capsule opened inward, so the increased cabin pressure prevented it from opening. It was another design change learned the hard way.