I don't really understand why Cannonical has decided to concentrate on adjusting Ubuntu to mobile, their main "selling" point is that you have a full desktop OS available on your phone. If they play their cards right, Ubuntu for Android [1] could pick up a huge percentage of OS market share in a couple of years when "desktop enabled" smartphones start replacing PCs for a lot of people.
No one else seems to be going in that direction, and by running alongside Android (for now) they wouldn't need to worry about the mobile part and have instant access to a huge existing userbase.
[1] http://www.ubuntu.com/devices/android