Because diesels (even modern) don't use throttle plates. They control power entirely through fuel metering, as predetonation is not an issue given the the more

Because diesels (even modern) don't use throttle plates. They control power entirely through fuel metering, as predetonation is not an issue given the the more robust cylinder design and nature of diesel fuel. Some diesels use so called J-brakes which effectively function as throttle plates to induce engine braking, but they are exclusively for slowing down the vehicle.