why is cruise control causing low mileage

Cruise control is a convenience only so you can rest your right foot / leg - rarely will it ever increase MPG due to the fact it tries to maintain a constant MPH and will utilize the throttle excessively if need be to do so - according to terrain .. I suppose if traveling on a relatively flat highway for 'several' miles it 'might' be as effective as the operator at maintaining throttle control as needed. My Chevy Blazer is the same - fact is they all are - it depends on the conditions / geolocation the vehicle is to be driven.