It's hard to explain to someone who hasn't used an e-reader, but e-paper screens just look more natural, they also don't require a backlight so they can be easier on the eyes. Combined with a matt display, you can comfortably use them in direct sunlight without having to faff about with brightness or finding shade.
Also e-readers have way less distractions than a phone.
Also e-readers have way less distractions than a phone.