Because it's not a backlight streaming through an LCD like most screens. It works by electrically moving little bits of actual pigment to the surface of the display. There is no backlight. The ambient light in the room (or, say, the reading light on an airplane) reflects off the pigments, just as it would on a real book. Take an LCD and an e-ink display outside on a sunny day, and the difference will be very apparent:
This article explains how e-ink works.
Trust me,
try one for a few minutes and you'll see what I mean. I had no idea what I was missing until I got one as a gift. Now I wish I had done it years ago. Another bonus is that it only uses electricity to
change the page, not to display content. Where an iPad will last 10 hours on a good day, my Kindle will last for
weeks of regular usage on a single charge.
It's not that this is a better or worse screen than an LCD. It's a different technology, better suited for a specific purpose (reading text). Just as a color LCD is better for watching videos.