Why is the region of Abyei contested?

The region of Abyei is contested as it is now considered in South Sudan, but is controlled by the North Sudanese government. The region of Abyei used to be a historical bridge connecting North and South Sudan, but since a terrible civil war erupted and the country recently split, it has now been granted special administrative status. There has now been a revised border, because violent disputes to who it belonged to erupted and making it smaller was agreed to by both sides.