The Southern states had homeguards and slave patrols to keep order. They operated more aggressivley following Lincoln's Emancipation Proclamation. More stringent curfews were imposed and movement of slaves was highly restricted. Another reason was that information about the deteriorating condition of the Southern military position may not have reached slaves, who were generally illiterate. Even when the approach of the Union was obvious, there was often uncertainty and fear. Furthermore, securing weapons, organizing and fighting are easier said than done. Therefore, slaves often remained passive and played a game of wait and see.

Source: http://www.georgiaencyclopedia.org/articles/history-archaeology/slave-patrols