It certainly doesn't help that Europe, particularly France, has spent the past two decades selling China the weapons and tech to do the invasion.
That said, Taiwan is a country of roughly similar population and economic clout as Australia. Standing by and ignoring an act of war on that scale would be devastating to international stability (and US security interests).
I would imagine that the precedent the US would set with not honoring is defense agreement would have enormous repercussions for its relationship and bargaining power with both South Korea and Japan.
That would tip the balance towards US intervention.
