A few issues:
PDFs: There's a bit of difficulty with these due to the inability to properly browse the things. Teasers are useful, but you never know what, exactly, is going to grab a consumer. Until there's a safe, secure, and widespread method for PDF sellers to SHOW their goods directly to people (Like having them available on the computers at bookstores for browsing through), they're going to be harder to market. Alas, the fact that there's a number of jerks out there who share PDFs with each other for free hardly helps.
Book Covers: For whatever reason, Hardcovers are sexy these days. This means higher costs for everyone involved. If people, en masse, expressed a soft cover fetish, this might possibly change for some books. This would save on space issues a bit, and cut a few costs here and there. It also means those books they get -seriously- used and start falling apart might be demanded second printings of.... muhahaha.
Frills: Sexy books are, well, sexier. Just like hard covers, this is fan taste. If we had more stoics in the community, maybe they'd save a few bucks on art costs and color inking. Not going to happen this century, so, eh.
Range of Competition: D&D is a form of entertainment, rather than 'literature' (especially nowadays, with the crunch focus). As such, with every new form of entertainment, D&D gets more competition. If publishers want to make money, they have to design something more alluring than XBox 2's three versions, PS3, the PC, and whatever the heck the latest gameboy is. And comics. And graphic novels. And 'normal' books. And manga. And anime. And Cartoon Network. And Comedy Central. And LARPing. And board games. And hunting/fishing. And dancing. And clubbing. And bar-hopping. and dating. And every other RPG. And older editions. And homebrewing. And hiking. And sports. And cable/satellite. And internet porn. That's a lot to deal with. Especially internet porn.

It takes a lot of sexing the books up to compete here, and sexing up costs money, because "Free Love" hasn't come back in to the fad system yet. (...maybe I shouldn't post after a date...)
There's a lot more than simple greed going here.