Because "Mix Tape" is copyrighted. ;-)
Basically, what Beats Music does (or tries to do) is be able to make personalized Playlists ("Mix Tapes") on the fly for whatever time of day or mood or activity strikes your fancy, and it will include (along with what you already know and/or own) new music you may not even be aware of or would think to add to the playlist. Each playlist being personally tailored to you at that particular moment in time. The mixture of human "curation" with complex algorithms is the key to Beats Music.
Add that "technology" to 800 million credit cards that Apple has on file via iTunes, the App Store, etc. and, well $$$
That is a pretty complicated explanation. If you asked your average consumer what Beats Music is they would say it is the same as Spotify, Pandora, etc: place to stream music.