Why the moon has no atmosphere?

The planet or satellite can bound the atmosphere i.e., mixture of gases if and only if the root mean square velocity of molecules of the gases is less than the escape velocity.
Escape velocity on the surface of the moon is small i.e. 2.38 km/sec as compared to r.m.s. velocity of gas molecules on the surface of the moon.
Therefore, all the gases escape from the gravitational field of the moon and hence is not present in the atmosphere.