It has largely to do with the fidelity of the microphones, not the quality of space-to-ground communications systems.
The astronauts wear wireless mikes that are as light and unobtrusive as NASA can make them. An FM signal goes from the astronaut to a communications device, then is radioed through a satellite system to the ground.
The microphones are fairly low fidelity. The better they are, the heavier they are, and on the shuttle, every ounce counts. NASA is not trying to obtain broadcast quality, and as long as the astronauts can be understood, that is enough.
Astronauts are given instruction in techniques for using a microphone, like not talking directly into it and avoiding the explosive is and bs that are likely to make the mike pop.
But they have got other things to worry about, and proper mike procedure is far down on the list.