Why does the earth have a liquid water supply and the moon does not

Because the gravity of the earth, unlike the gravity of the moon, is sufficient so that the earth's water does not evaporate or drain into space.

Explanation:

The most widely accepted explanation of how water on earth came about says that comets and asteroids - which have water in their constitution - bombarded our planet and left that element on its surface. Since the earth and the moon are so close together and occupy almost the same place in space, it is to be imagined that these comets and asteroids also hit the moon and leave water reserves there. However, the moon's gravity is much lower than the earth's gravity. This low moon gravity caused most of its water reservoir to evaporate or drain into space.

Because the Earth has biological ecosystems that ensure a supply of liquid water, inlcuding an atmosphere, living plant life and gaseous exchange systems that enable liquid water to be produced. None of this applies to the Moon.