Also remember that Rakuten's Kobo release was pretty botched from day one, which may have allowed Amazon extra time to polish their release. The electronics retailers are certainly still trying to push it as best they can, but the initial online reaction to the whole experience (updating the device, registering the account, tethering it to your desktop computer, and then after all that seeing the anemic bookstore selection) was overwhelmingly hostile.
Amazon's Japanese-language book selection isn't much more than Rakuten's, but even on an unregistered Kindle you've gotta love the ease of just entering your Amazon username and password, and hey, books!
