Why can't I use my Fitbit for Smart Lock?

I just updated my Nexus 5 to Lollipop, and the feature I'm most anticipating is Smart Lock — the ability to keep my phone unlocked when it's near me but still have strong protection if lost or stolen. I figured that since I have a Fitbit One in my pocket pretty much whenever I have my pants on, I'd use that as the "key". It syncs by Bluetooth, so I figured I'd be all set. However, when I go into Smart Lock in the settings and attempt to add a Trusted Device, it doesn't show up in the list. Why not?