I don't know if either of these is a DoD policy that might require what you have noticed, but they match my observations conceptually.
1) Aircraft that are used for combat (fighters and bombers) use UHF, while unarmed or defensively armed aircraft use VHF.
2) Light, high-performance aircraft use UHF, while heavy aircraft (except bombers) use VHF, likely so civilian aircraft are more aware of the presence of types that have similar speed and wake characteristics to commercial airliners.