Lower screen resolution requirements as well as physical screen size.
7" tablets are highly portable and a perfect size for mobile sketching and idea creation. For example, I just sold my iPad and switch to an HTC Flyer because of the portability. The Flyer is due for an upgrade to Android 3.2 very soon, but I won't be able to use Adobe Ideas because it has a screen size of 7" and a resolution of 1024x600px. I will likely use Sketchbook Pro because of this.