According to this article in the Daily Mail, it seems that there are a lot of songs, that weren't sold at least once. It says that the Long Tail theory of Chris Anderson predicts that about 80% of the songs should account for about 20% of the sales.
But just saying that it should work like this, doesn't make it so. For the Long Tail to work it should be easy to find, link to and buy songs that are not in the bestseller lists. If it's not easy to find a song, it won't be sold. If you can't link to a song, it won't be sold. If you can't listen to a song, it won't be sold. If you can't buy a song (for a reasonable price, in a reasonable way), it won't be sold. It should be easy to talk about and point people to those undiscovered songs.
Every roadblock on the way to buying a song is a place for a person to stop and go do something else. If you're running a business you should make it easier for people to find your product, not harder. Blame yourself, not other people.