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.