The World Politics Review quotes Africa Center Director J. Peter Pham on why despite the current stalemate between the government and rebel groups, only the rainy season is slowing fighting in South Sudan:
South Sudan is in the midst of its annual rainy season, which runs roughly from May until October. Given the country’s general lack of roads and other infrastructure, this hinders the ability of belligerents to move forces about. So while some fighting is happening at the moment—there have recently been some battles over control of oil fields in Unity and Upper Nile states—the clashes have been much more limited than during the dry season. But this relative calm is just a “time out” imposed by the weather. Sadly, the conflict will likely resume with full intensity once the rains end and the terrain becomes more passable.
The conflict has essentially been at a stalemate for some time. The Sudan People’s Liberation Movement (SPLM) government led by President Salva Kiir has survived, but does not have the wherewithal to defeat the various dissident movements of the so-called “Sudan People’s Liberation Movement-In Opposition” (SPLM-IO) that has grouped around former Vice President Riek Machar. The latter likewise doesn’t have the wherewithal to dislodge the regime from the towns it has retained control of.