Why is a 120w panel tilted toward the sun only making 15w at local noon?
Here is 100% of the information you need to answer the question with reasonable certainty:
Short answer: because only 15w is needed.
Longer answer: We know the controller is restricting power harvest since it’s running the panel at Vpanel == 21.6v, well above Vmp for a panel of that size. If the system were struggling to make the 15w Vpanel would be be Vmp, ~17-19v. We also wouldn’t be seeing strongly-defined shadows in the pic.
My guess is the controller is in float or late absorption with no meaningful loads attached.