I already have an universal computing device in my pocket.
Which is great, until it beeps or vibrates. Who could it be? Someone important? That email you're waiting for? Or just another spam message? Well, better take out the phone and see what's up. Repeat for 50 times a day. If this is you, then a smart watch is a handy thing. If this is not you, then you are not their target market, and good day to you.
Why would I want a device just for a single function?
With my pebble watch, it tells the time 24/7, shows messages, allows me to decline/accept calls, allows me to send "I'm busy" / "call me in 5", "I'll call you back", back to callers or messengers, lets me skip/pause/play tracks on my phone that's connected to a bluetooth speaker, etc etc. It also keeps a track of my daily exercise, sleep patterns and hear rate, and will buzz me if I'm being too slack, and that's when I go out and enjoy some nature.
Anyway, that's why a lot of people have these crazy things strapped to their wrists.