eReaders are really just a specialized version of a tablet.
Specialized devices have to be either really, really special or really, really cheap to make it in the marketplace. Otherwise, they get absorbed into a more widely usable device.
The smart phone is a good example. It can replace multiple specialized devices --- a phone, a watch, a camera, a flashlight, a GPS navigator, a TV --- and yes, even an ereader.
It may not be as good as a specialized device for any of these tasks but the real selling point is that it can do all of them reasonably well in a pinch. It packs a lot of functionality into one easily portable device.
EReaders are special, their screens make them an entirely different product category for an entirely different market.
That’s the marketing that they do, one gadget to replace them all. In reality I’ve an analog watch at my wrist, I’ve a powerful flashlight (mostly using my smartphone tho), I’ve a dslr camera and TV. One friend of mine his into boats and has multiple GPS devices properly adapted to boats. So the one fits all is good for generic users, but if you rely heavily of certain technologies, specialised gadgets are better, less distracting and more reliable.
eReaders are really, really special. It's the only usable electronic display when you're in a bright environment.