Because phone screens are too tiny, and LCD screens (on tablets or phones) cannot be read easily outside (and trying to read them in direct sunlight will kill the battery in jig time). Also e-readers are $100 to $250 cheaper than a tablet.
ETA: also, e-ink e-readers do a pretty good job of being a device for reading books. Every “distraction free” writing machine I’ve seen touted has suffered from major flaws making it not a very good device for writing words.
