Cloud may be a fad, but Ubuntu is making a lot of money on it. Mobile may not have been a fad, but Ubuntu was losing a lot of money on it.
Because of all the services Google provide on top of it. That's where the real value is, just like iOS, CyanogenMOD and co don't seem that popular. If Microsoft wants to make a come-back in the mobile space, that's where it needs to focus.