On Business: Format Wars
It should also come as no surprise that I have a mixture of digital and printed material. And a large part of why I might choose one over the other is availability. In some cases, it's an older work that no one has gotten around to digitizing yet. For example, Sensational She-Hulk #47 does not seem to be available anywhere, even though comiXology has the rest of that series' run. In other cases, it's a newer work that the author and/or publisher only released digitally because they wanted to gauge interest before committing to a large (i.e. expensive) print run.
(UPDATE: I had originally claimed "Clifford Meth's Comic Book Babylon was never released in print, I believe." Meth alerted me that I was wrong in that statement. It was printed by Aardwolf Publishing.)
Now there are all sorts of reasons why any given book might be released one way and not another. But, by and large, it usually boils down to: nobody in legal control of the contents thinks it would be profitable enough. That's not to say they're necessarily right or wrong, but that's their reading of the market. And despite my, or anyone else's, personal reading preferences it's hard to fault them for their decision. Even if you really, really, really want that big reference book available digitally so you don't have to lug the thing around while you're on a research trip, it's not reasonable to expect the publisher to make put in the time and effort to format a digital version just so it's easier on you. Likewise, you can't complain that a digital book hasn't been printed up just because the internet access at the favorite tree you like to read under is spotty.
As much as I enjoy and appreciate the benefits of digital publication, my book buying habits aren't going away any time soon because, until all these often obscure works that I'm doing research on are available electronically, I'm going to have to keep referring back to dead tree versions of them.