Patents. I can't find a source for this on the top of my head now, but "E-ink" is both a trademark and patented, strongly so. (It's not only a technology, it's the name of the company too, so you can't say "e-ink" without treading on their trademark. And they want you to say "E ink" and not "E-ink", apparently.)

Whereas there is a multitude of color display technologies with lots of prior art and overlap that would make patent disputes difficult, e-ink is so "special" in the way it works that it's easier for them to aggressively defend their patent, which means they take a huge skim off the top of anything that's produced. So there's lots of competition and innovation in the space of LCD and LED technology, because both those technologies are out of patent protection, but E-ink is a wasteland right now.

I think it's why it's so niche.