Here's why:

* Longitudinal waves are waves where the particles of the medium (in this case, air) vibrate parallel to the direction the wave travels.

* When the air cannon fires, it pushes a large amount of air forward, compressing the air in front of it. This compression travels outward as a wave of increased air pressure.

* The air molecules themselves are not traveling the distance of the wave; they are simply vibrating back and forth in the direction the wave is moving.

In contrast, transverse waves have particles vibrating perpendicular to the direction the wave travels. Examples of transverse waves include light waves and waves on a string.