As Lare said it is all about marketing. As much as we have been taught not to judge a book by its cover, there are ample research evidences showing that first impression does matter. In fact the Japanese publishing industry has been using manga style covers to attract the younger reading public for quite some time. Classical segmentation marketing for different target audiences isn't a new invention after all.
Ningen Shikkaku with a cover illustration designed by the mangaka of Death Note is a too well known example, and I am sure any self-respecting Japanese novel fan knows about it already. But even philosophical non-fictions like those by Descartes and Alain and are catching up with this trend.
Is it too much of a stretch to see a fictional series which already has (or going to have) its own manga, anime and movie adaptation to adopt a manga style cover?
Btw not exactly related, but I have written a mostly spoiler-free review of Another in my blog (in runes as usual). Please feel free to take a look.
