

Phones have been getting bigger over the years because people want to see more (whether for video, games, or just reading), the limit is that most people don’t want to lug a tablet around with them. A folding phone that provides a bigger screen and fits in your pocket doesn’t seem like a crazy concept to me, that’s why I’d quite like one (though I can’t justify the cost).
I assume low sales are because they’re ludicrously expensive, which makes sense for early technology. Apple are generally slow to adopt new things, they like quite a lot of polish (and to come up with some marketing so that they can claim they invented it!)