The reason for this is that we need Windows Phone 8.1 to support Bluetooth 4.0 (Scosche RHYTHM+), but we cannot use native Windows Phone 8.1, because *it does not allow background location tracking* :-(
For more on this, see
http://www.gps-sport.net/forums/thread/2838-0/
We hope that this will be possible with Windows 10 for Phones.