Here's why:

* Exosphere: Mercury does have a very thin layer of atoms called an exosphere, but it's so tenuous that it's practically a vacuum.

* Lack of Gravity: Mercury's small size and weak gravity mean it can't hold onto gases.

* Solar Wind: The constant stream of charged particles from the Sun (solar wind) constantly strips away any trace gases that might form.

While Mercury has a tiny exosphere, it doesn't have the density or composition to be considered a true atmosphere.