Why does Edge 130 (or the rest of Edge devices) needs power meter for VO2 measures

Why does Edge 130 (or the rest of Edge devices) needs power meter (plus HRM sensor) for VO2 measures while Garmin watches only needs a HRM sensor for the same type of activity (cycling)?

Why Edge 130 + Cadence sensor + HRM would not be enough to compute VO2 max?