Why was Georgia placed under military control for a second time during Reconstruction and its reentry to the United States further delayed?

Georgia was placed under military control for a second entry because the members of the General Assembly had expelled African American legislators.

Explanation:

In 1867, the state legislature of Georgia denied approving the Fifteenth Amendment according to which African-American citizens were allowed to vote. Thus, this prohibited the black settlers from voting even though the federal government allowed them for this right.

So, federal government formed pressure groups in the state which led to violence in the state and required military intervention to calm the situation. Finally, the amendment was approved in 1870 and 'Rufus Bullock' was elected the governor of the state.