Username or EmailPassword
Because design-by-committee has been time again shown to be slow and often substandard. Sometimes it works, sometimes not. It's true that this method rarely fails but it also rarely produces something innovative.
That's not even taking into account the typical knee-jerk reactions I would expect from the non-Ubuntu people in the open source world (which Ubuntu has plenty of experience with, by the way).
Arrogrant, bloody-minded, dictatorship (I'm purposely exaggerating) can be a good thing.