One reason could be because there are already other well-established solutions for integrating microcontrollers into SmartThings. Have you checked out my ST_Anything solution? You can implement basically any ST Capability (or multiples of each) with just a single simple Arduino sketch. Contact sensors, temperature sensors, humidity sensors, relays, momentary relays, water sensors, light level sensors, voltage sensors, smoke detectors, co detectors, etc… I’ve created an entire package to allow you to build your own “Anything” to meet whatever needs your project has.