> I get the joke, but color has been present in books (...)
That's like saying confortable cars have been present since their inception, and then present as a example a royal coach.
True, you technically had colors in books. Just like you had books with hardbacks with gold inlays. The fact is that the bulk of the books being published were not color books because that costs a premium to make and moreso to buy. Hence the default stabilized in softcover books with B/W print on flimsy paper.
With e-readers you do not get higher production costs, and you can just download your books and benefit from that. Some ebooks even ship with high resolution images where you can zoom in all you want or need.