Ask HN: Why are larger e-readers much more expensive than tablets?

Looking at larger e-readers (8-10inch) I am surprised how few options there are and the prices $300+. Yes the regular 6" Kindle is cheap but larger sizes are not.

I would rather read text books on an e-reader not a tablet.

The specs of most e-readers are low. Is the screen that costly? Is this the reason for them being that expensive? Or is it lack of demand?