Sudan

In 1881 Britain was in control of Sudan. Egypt gained joint control over Sudan in 1899. The British wanted to bring modern technology to Sudan for modernization and teaching the People English traditions. In Sudan the north is inhabited by those with Arab descent while the south is made up of those with African origin.
The British thought the south wasn’t ready for modern influence and so they closed the area from all outsiders. Only a few Arabs controlled the laws and businesses and Christian missionaries ran the schools and medical clinics. The full separation that was made caused the Arabs to believe that Sudan was an all Arab country.
Sudan became independent in 1956 with Britain handing control over to the Arabs and the separation of north and south became no more. The Arabs began to impose Islamic culture and religion on all Sudanese people. Southern Sudan resisted becoming Islam and a civil war broke out. The result left the south depopulated and displaced.
What I don’t understand is why the British didn’t let Sudan become two separate countries because that was how they had treated them when they isolated the south. Like most countries in Africa the Europe was the one who had determined counties borders at the end of the 1800’s.