I suspect that there are two primary audiences of music. One is people who listen to short songs individually while the other is people who prefer a series of longer pieces such as a symphony or concept album.
The first category is what dominates the folk, pop and rap charts while the second is dominant in classical music, progressive rock and to a lesser extent metal.
An interesting experiment would be to compare the single and album charts over the course of, perhaps, three years to see how much correlation there is between what artist are selling well.
If you have any thoughts on this subject please leave a comment below.