When you are just listening to music, your headset will be connecting via the A2DP Bluetooth profile (Advanced Audio Distribution Profile), which gives the best quality stereo audio connection. But, there is no possibility to use a microphone with this profile.
When you want to use the microphone, it has to switch to the HSP Bluetooth profile (Headset Profile). This allows bi-directional audio - i.e. a headphone and a microphone - but the quality will be lower.
This is because of the restricted bandwidth for passing data wirelessly via Bluetooth.