I think its an aesthetic decision. Steve Jobs would rather have a gorgeous full color, responsive display with limited battery life and wide functionality, than a merely decent color display with incredible battery life and optimized for reading. Just my guess.
I think the PixelQ tech is amazing, and hope Jepsen puts it on a bunch of devices. I'm just not sure Apple is the right fit for that kind of tech. My bet is AMOLED is Apple's next screen tech.