Because in due course we'll have Unity across all of these form factors. For now, however, Android is better on the phone UI than Unity, and so we're making it possible for people to start converging on the phone today with Android rather than waiting months for the Ubuntu phone UI itself.
You speak in the first person, are you involved with this project ?
We warn the reader in advance that the proof presented here depends on a clever but highly unmotivated trick. -- Howard Anton, "Elementary Linear Algebra"