I've thought about this a little, and it is probably a mix of a lot of things. Complexity, size, purpose, appearance, being the top.
Many of the 'wearables' are so complex. They try to 'fix' the problem with fewer buttons, and yet that only complicates things more. Size: Some are really HUGE. I'm used to wearing larger, thicker, dive watches, and even dive computers, so the Apple Watch is really thin by those comparisons, but for someone from the other side of the scale, women, people that wear 'dress' watches, they are seeing the bloated size of many of the wearables, and even though the Apple Watch hides it well, they and others are THICK, and large diameter/width, and heavy. (Clunky?). Purpose: I know many that will put on their Fitbit before they 'exercise', and take it right off afterwords. They view it as being very purpose exclusive. 'My Fitbit is for running, etc.' Appearance: Aside from size, many 'wearables' look just nasty...
But anyway...
I know people that hate wearing 'watches'. Say they are 'dated', and 'not cool', 'old fashioned'. There is that to contend with as well...
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If Apple Watch became independent of the iPhone then I think it would sell more. (If it was somehow able to run a duplicate SIM as what's in the iPhone so it's the same number and same data plan without additional costs and without the need to carry your phone, I think their sales would triple).
I was given a Shine as a gift, but I never wear it because it just doesn't have enough features. I don't own an Apple Watch because it doesn't have enough features. I'm hoping the Apple Watch 2 will have enough for me to pull the trigger, but as it stands, I can't justify buying it.
I guess that's why so many people buy the cheap fitness trackers: because all the wearable products out there are lacking so they figure they might as well get a cheap one.
But a standalone Watch would be thicker, and heavier, or the design trade-offs would make it unusable.