The iBookstore isn’t a failure because they haven’t seriously tried. However, the fact that Steve Jobs was interested in books at all is telling.
Apple has made no effort to seriously prioritize iBooks within their ecosystem. iOS devices don’t even ship with iBooks. All other Apple content stores are hardwired into iOS, even the upcoming Newsstand is included in iOS5. It is quite apparent that Apple only views iBooks only as one of hundreds of reasons to purchase an iOS device.