Supposedly (according to Recode.net) Beats music only has around 200,000 subscribers and most of them are free trial users from a AT&T deal. Doesn't sound a like a service Apple would want.
I'm not surprised. I'm a Beats subscriber and the only thing going for it is the (actually good) human-curated playlists. However, Spotify seems to have a bigger music catalog and music discovery is good enough.