We use CBR/CBZ for many reasons, portability being one, cost (free to create) being another. The lack of demand for PDF's is another reason (honestly, for every 1 person asking about PDF's, we have 100 or more visiting, downloading and enjoying the site as is). Also, to create PDF's it isn't free (I am sure there is some form of freeware that will do it). Plus, most PDF software by default will compress the images in some form or another, something that can have an adverse affect on the quality of the releases (with a CBR/CBZ it is just the raw images that are already compressed using JPG format).
Having come from Retromags did make it easier to settle on a format for our releases. There is a member on Retromags that is taking it upon himself to create PDF's of every release they put up. While he has not released download numbers to the public, I also haven't heard him shouting for joy at the demand either.