WiiU Sing E3 Preview

Since this generation seems to have had quite a vast protfolio of singing and dancing titles, it would have been strange if the WiiU platform didn't have one of its own. Sing uses the Gamepad as a karaoke display which allows the player to face the crowd who can in turn dance with the avatar on the TV. The weired thing is that the user in the preview had a microphone in her hand, our question is, why didn't they use the one on the gamepad?