Probably drowned in trying to support so many SKUs. Trying to segment their product beyond the ability of startup capital in a new and small market. Supply chain complexities, inventory burden, and too many subtly different models for a new (mainstream) customer to easily understand.

See here for a summary of all the models and options they were supporting: http://gadgetsandwearables.com/2017/10/23/best-pebble-watch/

Consumer hardware seems to be the Everest of startup endeavors. Congrats to the founder for moving forward in his career!