Why does the moon orbit earth instead of the sun?

Actually, as the moon orbits earth the earth orbits the sun bringing the moon along with it.

This is the reason and it does have something to do with masses an gravity, Both Newton and Einstein.

F =G(m1m2/r^2)

The force of gravity is inversely proportional to the mass of the two objects and the distance (r) between them.

The moon orbits the Earth, which orbits the sun. So in a way the moon orbits both the Earth and the sun. In physics an orbit is not necessarily circular or elliptical in shape.

The orbit of the moon around the sun is complicated as it is no longer a two body problem i.e. Along with the moon and the sun's gravity we will have to include the gravity of the Earth as well.

The moon's mass is pretty small as compared to both the Earth and the sun. However it is much closer to the Earth than the sun. Hence it is more influenced by the gravity of the Earth than the sun's.

Actually, the earth/ moon system is more "binary planets" than primary and secondary. Isaac Asimov wrote a wonderful essay "Mooning around" which showed that (then) unique in the solar system the earth/ moon pair were held more each to the sun than to each other. (since, it has been shown that Pluto/Charon have a simlar behavior).

It has something to do about the earth's mass i think.

Hahahahahaha,so you can have night and day in most of the known world.