A quadrotor that loses a motor is a stone. A hexrotor that loses a motor is still a flying machine — degraded, but flying. For the missions Vitruvius is designed for — inspecting wind turbines, carrying payloads over forest, working in places where a controlled descent is not always available — that distinction is the whole point.
Six rotors also let us run each motor at lower load, which means smaller propellers, slower tip speeds, and a far quieter machine than a comparable quadcopter at the same lift.