Question: Why doesn't the moon have an atmosphere?

Alan Fitzsimmons answered on 17 Mar 2014:

The simple reason there’s no large atmosphere on the Moon is that the Moon so small, its gravity is much weaker than the Earth’s. It’s gravity that holds the atmosphere down on Earth, just as it holds you and me. Gravity on the Moon is only one-sixth as strong as on the Earth. That means any gas would immediately drift off into space.

Strangely enough, your original question isn’t quite right. There IS a very very thin atmosphere on the Moon, discovered by the astronauts when they went there 40 years ago. It’s made by particles from the Sun hitting the rocks on the Moon and knocking out atoms, which then fly off into space. As this is happening all the time on the dayside of the Moon, there’s always a few atoms around, which makes a very very thin atmosphere. It’s much, much thinner than any air that you could breathe and only can be detected using sensitive instruments.