I don't think it's the microphone quality - even cheap mics will give reasonable voice audio quality. Also it can't be due to internet streaming quality, as the audio is pretty much the same quality as it's been since the beginning of the space program. The answer must be something to do with transmission techniques & protocols. Sounds like it's telephone-quality AM audio - whereas you want something more like FM quality. There must be reasons why AM audio is easier to transmit over long distances (and through the Van Allen belts - that could be a factor), but I don't know what those reasons would be. Like you, I would have thought digital FM transmission would be no big deal.

Maybe as Doug implies, it's simply that the existing quality does the job sufficiently, and good-resolution video is considered more important, and is thus allocated all the available digital bandwidth.