The answer is due to the distance from the Earth. The reason that stars appear to twinkle in the night sky is because of disturbance from the Earth's atmosphere. The stars do not actually really twinkle; they burn and produce light constantly like our Sun. But because they are so far away, their light as seen from the Earth is so small that particles in the Earth's atmosphere cause the light to be temporarily refracted and shifted in direction. This causes the twinkling effect. The reason that the Sun does not appear to twinkle from the Earth is because it is too close to be affected by this refraction.