Digital media lacks a spatial element
While digital media, such as e-readers, blog posts, etc., bring about a slew of benefits (enhanced UI, efficiency, cost effectiveness), there is a key element lost when compared to its analogue counterpart, the book: space.
Physical books provide the reader with a sense of space and progression in a number of ways: the number of pages behind and in front of the current pages being read give the reader a sense of how far through the book they are; how much has happened in the course of x number of pages, therefore how much more must be able to happen in the pages remaining? This is a poor example but there is an element of space that prompts metacognition.
Even something as simple as the turning of a page gives the brain just a few precious seconds to consider what has been read over the course of the page. In this way, the Kindle actually prevents the reader from contemplation, due to the speed of the page turn after the user touches the screen.