Mud is denser than sand and therefore can't be carried as far as sand, the latter of which can remain aloft for over thousands of miles. However, being denser, mud is able to cause direct damage like Sludge, for example. At least that's how I interpret it.
It makes sense until things like Confuse Ray versus Normals and Hypnosis versus Darks throw the logic out the window. :/It's probably a bug in Netbattle. Makes sense that it doesn't, as Glare is a normal move and all...