Because iPhones support AirPlay 2. Android phones don’t.

ungrettir.

All is not lost….

You could consider adding a Sonos Roam speaker to your existing Sonos setup and use a Bluetooth connection from your Samsung mobile to the Roam to play audio …and then ‘Group’ that new portable speaker with any/all of your current Sonos devices over WiFi. All those in the group will play the Bluetooth audio sent by the mobile.
 

Plus you then also have the added advantage of being able to use a portable rechargeable speaker in/around the home and whilst out and about with your phone.