Why can't I use my Fitbit for Smart Lock?
Bluetooth Low Energy devices can be used for Smart Lock, but they need to be paired at OS level and it seems that the Fitbit uses some kind of pseudo-pairing at app level. From the Fitbit manual:


  Unlike standard Bluetooth devices, like headsets, your Fitbit tracker will be set up directly through the Fitbit app and not through your Android device's Bluetooth pairing settings.