It all boils down to money. It is cheaper to publish an ebook than it is to publish a paper book. Although aviation education is a niche market, I am happy to say that my print books far outsell their ebook equivalents.
It may be cheaper to publish as an ebook for the publisher but they are forgetting about those of us who won't do ebooks. At least if they offered a print on demand option, they'd only print books that were bought and paid for and not have to worry about unsold copies. Otherwise, they are simply cutting off their nose to spite their face by refusing the option of getting my money.