Magazines apps are still too big for iPads because of all the images. Each iPad magazine takes up way too much storage, and people get turned off by the iPad magazines because of that. This is a problem specific to magazines because of all the imagery, especially for ads. Books don't have this problem, since they don't need ads.
The situation reminds me of early tiny MP3 players before the iPod came out with its hard drive that could store your whole MP3 collection. Magazines on iPads will start to happen once you can store a hundred or thousand issues on them at once.