Why is there no air on the moon

Today I just browsed a news on yahoo, about the brightest impact on the moon, and I also realized that the reason for the moon's many craters is it's absence of atmosphere which could resist an upcoming asteroids or meteors. And as what I know, it's because of the gravity that the earth has the ability to hold an atmosphere, and I just wonder, don't moon has enough gravity to at least hold even a thin layer of atmosphere?

Technically, the Moon has an extremely tenuous atmosphere but its molecules and atoms are so far apart that they don't collide. The reason is that the velocity necessary to escape from the vicinity of the Moon is well below the velocity of atoms and molecules of gas at the temperatures at the surface. However, there is generally just a little gaseous stuff for various reasons. It's so thin, though, that when the moon landings took place, the rocket engines on the lunar modules produced more gas locally than was present in the atmosphere before they arrived.

Gravity.

The Moon simply does not have the mass (and therefore the gravity) to hold on to an atmosphere.

As you reduce the mass of a planet or moon so it's gravity reduces. There comes a point when the gravity is so weak that its atmosphere is lost and boils off into space. Mars has a relatively thin atmosphere and is approaching that lower limit, Mercury is way below it and somewhere in between is the 'tipping point' where just the tiniest trace of atmosphere would be possible. The Moon is way below the minimum size.

It don't think it's known for certain, but the Moon probably never had much of an atmosphere to begin with. If it did, its gravitational attraction isn't great enough to resist the stripping action of the solar wind.
.
There *is* a very tenuous layer of ionized material that rises and falls slightly off the lunar surface with the Sun during a lunar day (28 Earth days), but that doesn't technically constitute an atmosphere.
.

Not for its size it won't