The facts on the ground are like this. The Dongolawis and Jaalis, and Shaygis became the elites of modern day Sudan through slave trading and forming their own armies ie the ansar army in the Mahdist state. Later on after that died down, they began educating themselves and creating/forming institutions in all sectors for themselves.
The Bejas in the east, and ths people from the Nuba mountains, and Southern Sudan, and the other non-Arab tribes in Darfur didn't do this as they too busy fighting amongst themselves and living a nomad lifestyle.
I also want to note out that the Arab tribes in Darfur were also marginalized for long periods too politically and economically speaking.