I find it surprising that there's 2 Australian cities on here but nowhere from the US. I've been under the impression Australia was similar to US in "Liveability" metrics, for good and ill.
My guess is that all USA cities were disqualified for possibilities of gun violence and lack of socialize medicine. I can't really disagree with that either.
They talk more about the methodology here, for what it's worth. I think crime is pretty heavily weighted, guessing that's what sinks a lot of US cities.
Osaka is a nice city but few Japanese would choose it as the no. 1 "livable" city in the nation. For a businessman it'd be much more convenient to live in some neighborhood city around Tokyo. To favor Osaka like that you'd need to mistake your personal preference as a universal measure.
It may not be sufficient, but it is necessary as a baseline precondition. Perhaps not all cities with good urbanism are "livable," but all "livable" cities must have good urbanism.