Linux has become the most popular operating system through forks like Android, Fire OS, Tizen, and Ubuntu, but now low-power wearable and Internet of Things (IoT) devices have bucked the trend.
Instead of utilizing the open source OS, these devices have found more success with real-time operating systems (RTOS) that function through a mobile OS, either iOS, Android, or in some cases Windows Phone.
See Also: Smartwatches fail to keep time despite wearables growth
Fitbit and Garmin are two of the best cases for RTOS, where the device provides a limited user interface for better power management