Because, of course, Android phones don't have Siri. The one feature you don't get when paired with an Android phone is the "Hey Siri" support.
Now, the phone and Powerbeats Pro are paired. Once you've gone through the pair step, you might see an option to allow access to your contacts/phones, which you'll need to make and receive calls. Now it should appear on the list, and you can select it.
It's available for free on the Play Store and will allow you to monitor your battery life and access a few key features Your first step - which didn't exist when Powerbeats Pro first launched - is downloading the Beats app to your phone.