Keeping them separate means people who may be interested in gaming but not music or vice versa get only the content and interaction they want, and Apple can fine tune both separately, each to better suit its own focus. Combining them together would mean Apple users only have one place to go to get everything related to their iOS (or Mac) experience.
Would Facebook be better if it was split into several distinct category sites? Would Twitter? Is Apple better able to compete with those unified, generic platforms by specializing and being distinct?
Many people cringe at the idea of even one more social network to join, how do you feel about two?