Android - 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.

In order for a Bluetooth device to work with smartlock, it needs to be constantly connected while it's within range of your phone. Dedicated smart watches, Bluetooth speakers, and your cars Bluetooth are constantly connected to your phone to pass data (calls, texts, music, notifications). For a lack of better term, the Bluetooth connectivity is "active" (always connected) for these devices.

Fitbit on the other hand only connects periodically and only for a few seconds to sync health data. So basically Bluetooth connectivity is in "sleep" mode (disconnected) until the timer activates Bluetooth to sync health data and then disconnects again after it's done.

There is an option for your fitbit device to notify you of incoming calls, however this still doesn't maintain Bluetooth connectivity. When you get a call, your phone temporarily activates Bluetooth to send the signal and then disconnects. This is why your fitbit gets a delayed notification instead of an instant notification like a true smart watch which has permanent Bluetooth connectivity.

Lollipop does not appear to allow Bluetooth Low Energy devices to be used for Smart Lock. I believe that Fitbits are devices of this type.

I have a similar issue with one of Fitbit's rivals - I've successfully set up Smart Lock with "normal" Bluetooth devices such as a speaker in my kitchen - i.e. ones that show a connection in the bluetooth settings - but I'm unable to do it with my UP24 band.