Basically, since there's so little atmospheric material, any particles that you do set in motion will travel very far in a straight line before they hit another

Basically, since there's so little atmospheric material, any particles that you do set in motion will travel very far in a straight line before they hit another, which is a lot faster than hitting a bunch of particles erratically. The catch is that you can only transmit very low frequency sounds - ti can be thought of like the variations in travel time for any individual particle drown out any high frequency signal.