Mightn't that just be because those courses are specifically to teach appreciation of those kinds of music? I expect it's probably possible to teach people who don't like rap, or country, to appreciate those genres; but because rap and country don't fit the shallow sense of high culture, no one is motivated to learn to appreciate them if they don't already. There is very little net benefit to learning to appreciate a new kind of music - there is abundant music in most genres, and one can easily fill one's ears with whatever one can most readily enjoy, so you probably don't get more total enjoyment from music by adding to your enjoyed genres. In the case of classical music, the benefit of learning to like it isn't really in the form of enjoyment of classical music; it's in the form of getting to sincerely claim to like classical music, and no longer being left out when highly cultured people discuss classical music.