- Author's Desire, at least in some genres. They have heard horror stories of the lower royalties (based on pre-Internet book contracts) for the format, "rampant" piracy losses, and of course some writers simply still don't "get" e-publishing, or otherwise associate as low class/amateurish compared to publishing a real physical object. Of course, this also works both ways – I've heard of a big name fiction author suddenly jumping on the bandwagon when she noticed a hated peer's latest release while sitting across the aisle from a Kindle-reader in some first class cabin somewhere.
- Publisher's Capacity. Not every print publisher today can support conversion to the wide variety of e-publishing formats, nor are they all aware of third-party solutions like Dan Pacheco's BookBrewer.com. Admittedly, most of these solutions (including Dan's) are more focused on targeting authors directly, though there's a lot of applicability for really small independent presses, as well.
- Publisher's Desire. Given a solidly established brick and mortar distribution channel, most publishers really didn't understand the true advantages of digital books until very recently. Big setbacks like the Borders implosion, as well as much harsher terms from big box stores like Wal*Mart and Costco have significantly changed this outlook, however, so I think we are seeing a pretty large uptick in availability across the board for new titles. The biggest laggard in my opinion are non-bestselling back catalog books, which have the most to gain from being perpetually "in stock" online… But from a publisher's perspective I understand why focusing investment on the biggest near-term wins makes the most sense on paper.
- Existing Book Contracts. Any print books published in at least the last 50 years or so usually include various types of ancillary rights clauses, for things like radio, tv, and movie rights, and more recently audiobooks[*]. As time has moved on, and lawyers have gotten progressively more evil, these clauses tended to get so broad and vague that any new technology/format – including Ebooks – usually sits under the control of the publisher for the term of the contract, or until certain conditions are met. This usually also means that e-publishing rights and royalties would need to be separately renegotiated (some big authors), or simply at the whim of the publisher (most everyone else), with the author usually receiving a much smaller cut of each sale, on what in theory is a much higher margin product.
[* Audiobooks are especially interesting to me, in that they suffer the contract and licensing restrictions of (a) books, (b) the recording industry, and (with the introduction of services like Audible.com) (c) digital distribution. How Audible, especially, ever got off the ground must've involved a huge team of lawyers, a vast portfolio of compromising hidden-camera photos, or both. ]
What factors determine whether a book is published electronically as well as traditionally?