There are various reasons why I don't like PDFs:
Mostly, I don't think the PDF format is that great for reading on screen. In most cases, the layout is a vertical rectangle, while the computer monitor is a horizontal rectangle. You generally can't see the whole page and have to use the mouse to move up and down. (I know some companies do use landscape format for easier reading, but they are in the minority).
It is also very very slow and on my computer, buggy - it frequently crashes or locks up. Which is ridiculous, as I can play most modern PC games. Used to be that you could turn off the large images (which is the big bog down), but they removed that option in Acrobat Reader 4 or so.
I've gotten a few offers of review copies of PDFs, but after the first one, I've always turned them down, because I really can't read them well enough to use or review properly.
I didn't really notice this until I got the print version of Blood & Space. Being able to sit down and read the book from cover to cover, I realized I missed a whole lot of stuff in the PDF.
I've always thought PDF was more for desktop publishing, as it is good for printing. I'd like to see companies use a format for reading on a monitor, like microsoft's Lit format. That's very easy on the eyes, very stable, and doesn't stress the computer a lot. Or just in Rich Text or Word format. Just having one column is so much easier to read than a PDF with 2 or 3.
Also, as there is basically only one source of PDFs, and no secondary market, the price is constant.
I probably buy most of my RPG stuff online from either a discount seller (like FRP Games), or ebay.
Generally speaking, on Ebay, as a product gets older, the more the price drops. You can get most older d20 stuff for 20%-35% of cover price or so on Ebay. For instance, I just got a bunch of S&S hardbacks (including Tome of Horrors and Necropolis) for $10 each (with shipping).
Given there is a ton of old stuff I want that I don't have, and can get cheap, it's hard to justify buying a PDF. Unless it's something really cool and not available in any other format.
Anyway, I think that once portable ebook readers or somesuch get cheap and popular, then e-publishing RPGs will hit it big. But until then, it's always going to be a somewhat niche market...