They do have some series available in eBook format but digital distribution is fairly new territory for publishers particularly for Viz who is usually a very conservative company when approaching new media. Seriously, they've just announced publishing yaoi when that type of genre has already been published by other manga publishers for years now.

Also, I think they're doing it this way so they at least have some control over their issues and clamping down on any piracy that may ensue if they did make it available as downloads for eReaders. :< ...But if you're a paying subscriber, give them your input and ask for Kindle or Nook downloads and maybe they'll listen.