Here's why:
* "Looking good" is a past progressive tense, indicating an action that was happening over a period of time in the past.
* "Was good" is a simple past tense, indicating a state of being that existed in the past.
The context likely implies that someone's appearance was pleasing, and "looking good" conveys that action of being visually appealing, while "was good" would imply a general state of being good.