Fundamentally exploration is much more important for music than it is for books due to the time it takes to consume a single unit. Either way, usually the most important part is showing that a customer is willing to make the jump from $0 to $0.01 and there are numerous music services that have shown that pirates are willing to make that leap in the right circumstances.
Which are all subscription-based and not unit-based. There is no cost to explore and listen to random songs when you've already paid your subscription anyway. Switch to unit-based and suddenly way less exploration. So it's unit-less pricing + convenience.