Probably because the head unit only knows how to deal with BT audio. Has no built in functionality for anything else. There's no software on the HU. As example, my hearing aids have a BT functionality, but only with an app specifically developed for the iPhone. The BT connection is only a method to get data between devices. I have a Google phone and as the HA manufacturer has not developed an app for Android, I cannot control anything on my HA's.