I guess the medium itself inexplicably aids or impedes duplication, and consequently aids or impedes convenience for the consumer.
Having the SD card medium would make sense to those who want something tangible (albeit physically small) for their money. For the lifespan of this card, the book would be useable, and could duplicate the funtionality of a book, except for the reading part.
But it has its downsides as brought up succintly by Luag!
I personally would choose digital format that can be moved around as opposed to one stuck in a card.