Why was there conflict between northern and southern Sudan?

In 2005, a Comprehensive Peace Agreement was signed by the Sudan People’s Liberation Army and Khartoum. This ended the civil war and allowed for an election and eventual South Sudanese independence in 2011. However, its lack of implementation has sparked conflict in Abyei, South Kordofan, Blue Nile and indirectly affected conflict in Darfur where oil was rich.