Why are smart bands not more of a thing?

When Pebble was a going concern there were a couple of smart bands that appeared to add functionality to the Pebble watch. I liked the idea of these although I’m not sure that they were particularly successful.

The Apple Watch has had a couple of similar add ons with bands that add video capability and one that adds gestural control. I looked at both and decided that, whilst they were both interesting, they were both bulky and I knew that in the end I wouldn’t use them and they’d end up on eBay.

Now I wonder why these haven’t become more mainstream? It would seem like an obvious place to put more functionality and controls, but no one seems to be doing that. I do wonder if a future Apple Watch will bring that kind of functionality. One thing is for sure though. They’ll have looked at it, or they are looking at it.