There are really only two reasons why a book is not an ebook, in my experience (I don’t speak for my employer, this is just my opinion).
1. Cost. To convert an electronic file into an EPUB, which is the format most ereaders take, costs about $200. Publishers can’t afford to convert backlist books that they have electronic files for if they aren’t going to quickly sell enough to earn that back. If there’s no electronic file, then the cost goes up considerably–you have pay to scan the book and pay to proofread the scan and then format it for EPUB. Some publishers skip the proofreading. You may have noticed. Some publishers are able to add staff to do this. You’re probably not complaining about not being able to find their books to read on your device.
2. Rights. Older contracts are not clear and finding the rightsholders to get things clarified is time-consuming (which means staff-consuming). Many authors held on to their electronic rights, so publishers can’t make ebooks and the authors don’t know how (and see 1). If publishers want to go back and renegotiate, that’s also time and money.
Those are the two main reasons–they don’t have the money to make an ebook or they don’t have permission.
There’s a third, minor, reason–but it is also a biggie and that has to do with why some books are available on some devices but not others.I don’t want to get into specifics for obvious reasons but here it is: How many of these vendors want to to interact with publishers financially is not possible for all publishers and many of the vendors seem unable to respond in a flexible manner. Their reporting doesn’t fit with the reporting publishers need; some, for instance, can’t tell you which ISBNs what money goes to. Some can’t write you a check and the Publisher may be unable to let them dump money into a bank account without warning. Some need you to be able to agree to a EULA and you may be affiliated with your a parent institution in a way that means you HAVE to have a negotiable contract (if your parent institution is a state government, you may be legally required). Some want you to sign confidentiality agreements and, again, if you’re affiliated with a state government, you might be bound by sunshine laws that forbid that. And you’d be surprised by which behemoths actually return phone calls and emails and have helpful people you can talk to and which seem to think that a FAQ is enough.
So let me just say that, if you look and say “Well, this is available on x and y, but I have a reading device from z, what’s the problem?” The problem could be that z thinks the FAQ solves all and therefore emails never have to be answered.
Not that I have any person experience with that.