If you want an e-ink device using 20 year old technology you can get a used one for cheaper than any would be competitor can produce one.

If you want one using anything developed in the past 5 years...oh, look, that's why eink still has a research division; they've been slowly improving things (not as fast as actual competition would cause, but enough to make it hard for someone to just leapfrog them using seed capital) and so would-be competitors are now running up against patents with 15+ years left.