South Sudan is country code 211.(The plus sign means insert your international access prefix here. From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)
The broad answer to that question is that Sudan and its southern half just dont have enough in common. For instance, the north is primarily Arabic in culture, whereas the south is tribal black animists.