going uphill offers a consistent resistance - gravity and rolling resistance don't change much unless the grade does. You can get into a rhythm and evenly put out power.
on flats or downhill, the majority of the resistance is from the air. Winds, even on a calm day are variable, meaning your resistance is always changing. It seems much harder to maintain a certain power because it really is hard without constant resistance.
don't believe me? hop on a trainer. With just a bit of practice, you can hold +/- 5w with ease