Whilst the product makes sense as a member of Nest’s range, smoke detectors are a much harder sell than thermostats. Smoke detectors are largely passive devices. Nest trades on annoyances … and smoke detectors annoy you on a less frequent basis than thermostats.
This is compounded by the fact that thermostats are inherently good demos, either on video or in a store, whereas a smoke detector just sits there like a brick.
The most common scenario when consumers will buy a new smoke detector will be when their current smoke detector comes to the end of its life. Instilling an upgrade (not merely replacement) pattern into the general public for a smoke detector is an uphill battle.