As a disclaimer before I say anything at all, I don't work in publishing or the games industry and know very little about economics, so if you do know these things and what I say is ridiculous, I'd love to hear why.
Wizards of the Coast stopped selling PDF copies of D&D books a few years ago, despite the fact that demand is there, presumably because they were worried about piracy. Even a PDF that's somehow locked or protected can be broken into and released DRM-free, so Wizards' approach was to... stop selling PDFs.
There are a lot of people who either prefer PDFs, or would like the option of owning both print and PDF copies of their books (I personally like to have both because as a DM, it's hard to carry ten or more hardcover books to a session that's not at my place).
And here's the key thing: not offering official PDF copies does not mean that PDFs don't exist. It's a simple matter to scan and compile a physical book into a digital copy and (illegally) distribute it for free, and if you do a quick search on The Pirate Bay, you can see that it's being done.
So my question is this: if people are going to distribute PDFs anyway, why not sell legal ones to capture at least some of that market?
PDFs are relatively small files, so hosting is trivial. Not to mention that you don't need a warehouse like you do with physical books - one file is enough, and you send out copies to buyers with no printing costs or volume requirements.
Bandwidth may be more expensive, but again, they're smallish files.
Actually, a fantastically elegant solution would be to include a code in each physical book, and through the Wizards website, you can redeem the code for a PDF copy of that book. It wouldn't necessarily increase sales by vast margins, but it would be an excellent gesture and acknowledgement of the digital age and the difficulty of carrying around dozens of hardcover books.
EDIT: it's been pointed out to me that people could easily steal a card or snap a picture of the code unless the books are wrapped in plastic... which would increase manufacturing costs. Perhaps they should just sell PDFs on the website again.
As I said above, if this doesn't make sense for business reasons, I'd like to know why. I'd also like to know how D&D fans feel about PDFs in general - is there a big enough demand for PDFs for this to be worth Wizards' time, for example?