Question

why Pluto is not as bright as Neptune.

Answer 1

Answer:

Neptune is brighter than Pluto because it is larger and (usually) closer to earth. There isn't enough differences in the albedo or "reflectivity" of the two planets to offset Neptune's size advantage. Even when Pluto slides inside the orbit of Neptune, which the dwarf planet does during part of its orbit of the sun, its smaller size does not permit it to compete with Neptune for brightness.