As to "why": I'm not interested at all in Garmin's sports and health features and their cloud stuff. Pebbles work well, get out of the way and have a nice, friendly and slightly quirky interface. There are many apps and watchfaces out there. As of the new ones, the OS is open source. Reasons enough for me. ;)
I likely will continue to use my Garmin watch because I rely heavily on the fitness stuff that they do so well, but outside of that the Garmin is "just OK."
If you want a fitness tracker that is also a smartwatch the Garmin is a great choice. If you want a smartwatch and don't care about fitness tracking then you're wasting a lot of money on stuff you don't care about with Garmin, for just an OK and extremely locked down device.
Otherwise I agree. I would love an opensource fitness watch though, but I understand why this is not (yet?) possible.