Tyler Cowen of Marginal Revolution offers some thoughts in response to a reader about the different lengths of albums across different genres like rap, metal, prog, and jazz. The simple and immediate answer is that the length has a lot to do with the written or unwritten formulas of a particular music genre. But Cowen pointed out that it also has a lot to do on how varied the audience is:
If you wanted a somewhat strained explanation, you could argue that the longer CD is a more bundled product and it will make economic sense as a form of price discrimination, the more varied the valuations of the audience. This would require that rap CD buyers have a higher variance of marginal valuation.
That makes total sense because the hip-hop and rap audience is probably the most varied out there right now.