Any way the second reason why people lost interest in the creation of PGDCE is that at the time when we were still in the process of creating documentation the Castle Engine was published. So many people would rather go making new games on existing engine rather than making the engine itself.
That is a strong point, especially by this day and age where game engine like Unity and Unreal exists.
Many people doesn't care anymore about what is under the hood.

I don't advocate for these tools, but it is a reality.