Contrast ratios likely play into this as well. I ran across some research a while ago that found that a contrast ratio of about 8:1 is ideal for readability. E-

Contrast ratios likely play into this as well. I ran across some research a while ago that found that a contrast ratio of about 8:1 is ideal for readability. E-ink is pretty close to that optimum (usually around 10-15:1 these days). Whereas for conventional screens contrast ratios keep being pushed higher and higher (1000:1 is pretty typical). While this is great for video, it makes sense that at some point it becomes tiring for the eye when viewing black text on a white background (imagine trying to focus on an object superimposed against the sky on a blindingly bright summer day).