Because Android's UI framework is quite behind that of iOS and even that of Windows Phone's. Forking and fixing the UI framework is not doable.
Besides Ubuntu is in OS business, they would rather offer an alternative to Android.
Besides Ubuntu is in OS business, they would rather offer an alternative to Android.
The mobile Ubuntu for Phones is actually entitled "Ubuntu for Android", and is billed as "The full PC desktop for multi-core Android phones".
That seems to stand in stark contrast to literally everything you said.