The tablet version came out some months before the phone version, if I recall.
Not sure why the decision was made to make them two separate apps. I can say that they're practically the same app, just that one's UI is designed for tablets while the other is designed for phones. You can't use the tablet version in portrait orientation on a tablet. Likewise, you can't use the phone version in landscape orientation. This all may change. Autodesk's SketchBook tablet app could only work in one orientation (landscape) until a few versions later where it could finally change to either landscape or portrait.