From Twitter:

Math: 58% of install base on iOS 8.4 * 500 million install base = 290 million. 11 million trials/290 million = 3.7% testing Apple Music.

Only 2.2% of iPhone owners are trying Apple Music. 4% of people that upgraded to the latest software are trying it.

Lots of people haven't even tried it yet (those % are even less because the iOS install base + Macs is more than 500 million), plenty of room for growth.

Also, it's not available in every country that sells iPhones.