Partially because Amazon lost to the Publishers in 2014-15 - https://www.nytimes.com/2014/11/14/technology/amazon-hachett... ? All ebook prices jumped after that.

I am not sure why no one is correlating the price increase to the the drop in Ebook sales, especially since Amazon at that point pointed out exactly this as a consequence - since ebooks have to compete with other digital media.