Here's why:

* Planets orbit stars. The Moon orbits Earth, which in turn orbits the Sun.

* Planets must clear their orbital path of other objects. The Moon shares its orbit with other celestial bodies, like asteroids.

* Planets have sufficient mass for their gravity to pull them into a round shape. The Moon is round due to its own gravity, but it doesn't have the same gravitational dominance as a planet.

So, while the Moon is a large and important celestial body, it doesn't meet the criteria to be classified as a planet.