Because, at least in my opinion, if you set something as your backdrop it should be at least a little bit important because it is a part of what informs your characters and the story you want to tell. Altman can swear all day and all night it isnt a movie about country music (and obviously it isnt JUST about country music) but the characters feelings and motivations are partially shown through their music, especially since most of the actors wrote their own music for the film. I think if one wants to argue for the film and remove the music than they will lose a large portion of why and how the characters interact, which is often times the most important thing in Altman's films, or anyone elses for that matter.