As the citric acid cycle proceeds from the entry of acetyl CoA to the production of succinate, two carbons enter the cycle and two carbons are released as CO2. Why is the cycle not considered complete at this point?

Because ATP is not produce yet.

Explanation:

The cycle is not considered as complete because the main product i. e. adenine tri phosphate (ATP) is not produced yet. The main product of cellular respiration is the production of energy in the form of ATP with the help of oxygen whereas water and carbondioxide are the byproduct as well as waste material which can be removed with the help of breathing so that's why the cycle is considered as incomplete.