In much the same way that NVIDIA may have felt that EGL was the better choice.
However none of your description of the way things are explains why NVIDIA couldn’t have made their own libgbm that matched the symbols of mesa and worked on standardizing the api by de facto.
Mesa did discuss EGL but felt it wasn’t the right choice. https://mesa-dev.freedesktop.narkive.com/qq4iQ7RR/egl-stream...
In much the same way that NVIDIA may have felt that EGL was the better choice.
However none of your description of the way things are explains why NVIDIA couldn’t have made their own libgbm that matched the symbols of mesa and worked on standardizing the api by de facto.